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>

bitchat/_PreviewHelpers/PreviewKeychainManager.swift

@@ -41,6 +41,19 @@ final class PreviewKeychainManager: KeychainManagerProtocol {
         storage["identity_noiseStaticKey"] != nil
     }
 
+    // BCH-01-009: New methods with proper error classification
+    func getIdentityKeyWithResult(forKey key: String) -> KeychainReadResult {
+        if let data = storage[key] {
+            return .success(data)
+        }
+        return .itemNotFound
+    }
+
+    func saveIdentityKeyWithResult(_ keyData: Data, forKey key: String) -> KeychainSaveResult {
+        storage[key] = keyData
+        return .success
+    }
+
     // MARK: - Generic Data Storage (consolidated from KeychainHelper)
 
     func save(key: String, data: Data, service: String, accessible: CFString?) {