fix(security): improve keychain error handling [BCH-01-009]

Add proper error classification to distinguish expected states (item not
found) from critical failures (access denied, storage full) and
recoverable errors (device locked, auth failed).

Changes:
- Add KeychainReadResult and KeychainSaveResult enums with error types
- Add getIdentityKeyWithResult/saveIdentityKeyWithResult protocol methods
- Implement retry logic with exponential backoff for transient errors
- Add consistent SecureLogger logging for all keychain operations
- Update NoiseEncryptionService identity loading to handle errors gracefully
- Update MockKeychain and TrackingMockKeychain with error simulation
- Add comprehensive KeychainErrorHandlingTests (18 new tests)

Security: Applications can now properly detect and respond to critical
keychain failures, improving resilience during device lock states and
providing actionable diagnostics for access issues.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

bitchatTests/KeychainErrorHandlingTests.swift

@@ -0,0 +1,216 @@
+//
+// KeychainErrorHandlingTests.swift
+// bitchatTests
+//
+// This is free and unencumbered software released into the public domain.
+// For more information, see <https://unlicense.org>
+//
+// BCH-01-009: Tests for proper keychain error classification and handling
+
+import Testing
+import Foundation
+@testable import bitchat
+
+struct KeychainErrorHandlingTests {
+
+    // MARK: - Error Classification Tests
+
+    @Test func keychainReadResult_successIsNotRecoverable() throws {
+        let result = KeychainReadResult.success(Data([1, 2, 3]))
+        #expect(result.isRecoverableError == false)
+    }
+
+    @Test func keychainReadResult_itemNotFoundIsNotRecoverable() throws {
+        let result = KeychainReadResult.itemNotFound
+        #expect(result.isRecoverableError == false)
+    }
+
+    @Test func keychainReadResult_deviceLockedIsRecoverable() throws {
+        let result = KeychainReadResult.deviceLocked
+        #expect(result.isRecoverableError == true)
+    }
+
+    @Test func keychainReadResult_authenticationFailedIsRecoverable() throws {
+        let result = KeychainReadResult.authenticationFailed
+        #expect(result.isRecoverableError == true)
+    }
+
+    @Test func keychainReadResult_accessDeniedIsNotRecoverable() throws {
+        let result = KeychainReadResult.accessDenied
+        #expect(result.isRecoverableError == false)
+    }
+
+    @Test func keychainSaveResult_successIsNotRecoverable() throws {
+        let result = KeychainSaveResult.success
+        #expect(result.isRecoverableError == false)
+    }
+
+    @Test func keychainSaveResult_duplicateItemIsRecoverable() throws {
+        let result = KeychainSaveResult.duplicateItem
+        #expect(result.isRecoverableError == true)
+    }
+
+    @Test func keychainSaveResult_deviceLockedIsRecoverable() throws {
+        let result = KeychainSaveResult.deviceLocked
+        #expect(result.isRecoverableError == true)
+    }
+
+    @Test func keychainSaveResult_storageFullIsNotRecoverable() throws {
+        let result = KeychainSaveResult.storageFull
+        #expect(result.isRecoverableError == false)
+    }
+
+    // MARK: - Mock Keychain Error Simulation Tests
+
+    @Test func mockKeychain_canSimulateReadErrors() throws {
+        let keychain = MockKeychain()
+
+        // Simulate access denied error
+        keychain.simulatedReadError = .accessDenied
+        let result = keychain.getIdentityKeyWithResult(forKey: "testKey")
+
+        switch result {
+        case .accessDenied:
+            // Expected
+            break
+        default:
+            throw KeychainTestError("Expected accessDenied, got \(result)")
+        }
+    }
+
+    @Test func mockKeychain_canSimulateSaveErrors() throws {
+        let keychain = MockKeychain()
+
+        // Simulate storage full error
+        keychain.simulatedSaveError = .storageFull
+        let result = keychain.saveIdentityKeyWithResult(Data([1, 2, 3]), forKey: "testKey")
+
+        switch result {
+        case .storageFull:
+            // Expected
+            break
+        default:
+            throw KeychainTestError("Expected storageFull, got \(result)")
+        }
+    }
+
+    @Test func mockKeychain_returnsItemNotFoundForMissingKey() throws {
+        let keychain = MockKeychain()
+        let result = keychain.getIdentityKeyWithResult(forKey: "nonExistentKey")
+
+        switch result {
+        case .itemNotFound:
+            // Expected
+            break
+        default:
+            throw KeychainTestError("Expected itemNotFound, got \(result)")
+        }
+    }
+
+    @Test func mockKeychain_returnsSuccessForExistingKey() throws {
+        let keychain = MockKeychain()
+        let testData = Data([1, 2, 3, 4, 5])
+
+        // First save the key
+        _ = keychain.saveIdentityKey(testData, forKey: "existingKey")
+
+        // Now read it back
+        let result = keychain.getIdentityKeyWithResult(forKey: "existingKey")
+
+        switch result {
+        case .success(let data):
+            #expect(data == testData)
+        default:
+            throw KeychainTestError("Expected success, got \(result)")
+        }
+    }
+
+    @Test func mockKeychain_saveWithResultStoresData() throws {
+        let keychain = MockKeychain()
+        let testData = Data([10, 20, 30])
+
+        let saveResult = keychain.saveIdentityKeyWithResult(testData, forKey: "newKey")
+
+        switch saveResult {
+        case .success:
+            // Verify data was stored
+            let readResult = keychain.getIdentityKeyWithResult(forKey: "newKey")
+            switch readResult {
+            case .success(let data):
+                #expect(data == testData)
+            default:
+                throw KeychainTestError("Expected to read back saved data")
+            }
+        default:
+            throw KeychainTestError("Expected save success, got \(saveResult)")
+        }
+    }
+
+    // MARK: - NoiseEncryptionService Integration Tests
+
+    @Test func noiseEncryptionService_generatesNewIdentityWhenMissing() throws {
+        let keychain = MockKeychain()
+
+        // Create service with empty keychain - should generate new identity
+        let service = NoiseEncryptionService(keychain: keychain)
+
+        // Should have generated and saved keys
+        #expect(service.getStaticPublicKeyData().count == 32)
+        #expect(service.getSigningPublicKeyData().count == 32)
+
+        // Keys should be persisted
+        let noiseKeyResult = keychain.getIdentityKeyWithResult(forKey: "noiseStaticKey")
+        switch noiseKeyResult {
+        case .success:
+            // Expected - key was saved
+            break
+        default:
+            throw KeychainTestError("Expected noise key to be saved")
+        }
+    }
+
+    @Test func noiseEncryptionService_loadsExistingIdentity() throws {
+        let keychain = MockKeychain()
+
+        // Create first service to generate identity
+        let service1 = NoiseEncryptionService(keychain: keychain)
+        let originalPublicKey = service1.getStaticPublicKeyData()
+        let originalSigningKey = service1.getSigningPublicKeyData()
+
+        // Create second service - should load same identity
+        let service2 = NoiseEncryptionService(keychain: keychain)
+
+        #expect(service2.getStaticPublicKeyData() == originalPublicKey)
+        #expect(service2.getSigningPublicKeyData() == originalSigningKey)
+    }
+
+    @Test func noiseEncryptionService_handlesAccessDeniedGracefully() throws {
+        let keychain = MockKeychain()
+        keychain.simulatedReadError = .accessDenied
+
+        // Service should still initialize with ephemeral key
+        let service = NoiseEncryptionService(keychain: keychain)
+
+        // Should have an identity (ephemeral)
+        #expect(service.getStaticPublicKeyData().count == 32)
+        #expect(service.getSigningPublicKeyData().count == 32)
+    }
+
+    @Test func noiseEncryptionService_handlesDeviceLockedGracefully() throws {
+        let keychain = MockKeychain()
+        keychain.simulatedReadError = .deviceLocked
+
+        // Service should still initialize with ephemeral key
+        let service = NoiseEncryptionService(keychain: keychain)
+
+        // Should have an identity (ephemeral)
+        #expect(service.getStaticPublicKeyData().count == 32)
+    }
+}
+
+// Helper error type for tests
+private struct KeychainTestError: Error, CustomStringConvertible {
+    let message: String
+    init(_ message: String) { self.message = message }
+    var description: String { message }
+}