fix: clear DH shared secrets after Noise handshake operations

Added secureClear() calls for all 6 DH operations in NoiseProtocol.swift
to properly clear sensitive shared secrets from memory after use:

- writeMessage(): .es initiator/responder, .se initiator/responder
- performDHOperation(): .ee and .ss operations

This fixes a forward secrecy vulnerability where shared secrets could
persist in memory after handshake completion.

Also adds:
- TrackingMockKeychain to count secureClear calls in tests
- 3 new tests verifying secureClear is called during handshake

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

bitchatTests/Mocks/MockKeychain.swift

@@ -65,3 +65,90 @@ final class MockKeychain: KeychainManagerProtocol {
 
 /// Typealias for backwards compatibility with tests using MockKeychainHelper
 typealias MockKeychainHelper = MockKeychain
+
+/// Mock keychain that tracks secureClear calls for testing DH secret clearing
+final class TrackingMockKeychain: KeychainManagerProtocol {
+    private var storage: [String: Data] = [:]
+    private var serviceStorage: [String: [String: Data]] = [:]
+
+    /// Thread-safe counter for secureClear calls
+    private let lock = NSLock()
+    private var _secureClearDataCallCount = 0
+    private var _secureClearStringCallCount = 0
+
+    var secureClearDataCallCount: Int {
+        lock.lock()
+        defer { lock.unlock() }
+        return _secureClearDataCallCount
+    }
+
+    var secureClearStringCallCount: Int {
+        lock.lock()
+        defer { lock.unlock() }
+        return _secureClearStringCallCount
+    }
+
+    var totalSecureClearCallCount: Int {
+        return secureClearDataCallCount + secureClearStringCallCount
+    }
+
+    func resetCounts() {
+        lock.lock()
+        defer { lock.unlock() }
+        _secureClearDataCallCount = 0
+        _secureClearStringCallCount = 0
+    }
+
+    func saveIdentityKey(_ keyData: Data, forKey key: String) -> Bool {
+        storage[key] = keyData
+        return true
+    }
+
+    func getIdentityKey(forKey key: String) -> Data? {
+        storage[key]
+    }
+
+    func deleteIdentityKey(forKey key: String) -> Bool {
+        storage.removeValue(forKey: key)
+        return true
+    }
+
+    func deleteAllKeychainData() -> Bool {
+        storage.removeAll()
+        serviceStorage.removeAll()
+        return true
+    }
+
+    func secureClear(_ data: inout Data) {
+        lock.lock()
+        _secureClearDataCallCount += 1
+        lock.unlock()
+        data = Data()
+    }
+
+    func secureClear(_ string: inout String) {
+        lock.lock()
+        _secureClearStringCallCount += 1
+        lock.unlock()
+        string = ""
+    }
+
+    func verifyIdentityKeyExists() -> Bool {
+        storage["identity_noiseStaticKey"] != nil
+    }
+
+    func save(key: String, data: Data, service: String, accessible: CFString?) {
+        if serviceStorage[service] == nil {
+            serviceStorage[service] = [:]
+        }
+        serviceStorage[service]?[key] = data
+    }
+
+    func load(key: String, service: String) -> Data? {
+        serviceStorage[service]?[key]
+    }
+
+    func delete(key: String, service: String) {
+        serviceStorage[service]?.removeValue(forKey: key)
+    }
+}