Fix all compilation errors and warnings in test suite

- Fixed mock service property overrides to match base class properties
- Added missing CryptoKit imports where needed
- Fixed immutable property assignments by creating new instances
- Replaced XCTAssertThrows with XCTAssertThrowsError
- Fixed DeliveryAck serialization method names (serialize -> encode)
- Fixed unused variable warnings
- Ensured all BitchatPacket modifications create new instances
- Fixed BitchatMessage property mutations by creating new instances

All test targets now build successfully for both iOS and macOS platforms.

bitchatTests/TestUtilities/TestConstants.swift

@@ -28,6 +28,5 @@ struct TestConstants {
     static let testMessage3 = "This is a test message"
     static let testLongMessage = String(repeating: "This is a long message. ", count: 100)
     
-    static let testRoomID = "test-room"
     static let testSignature = Data(repeating: 0xAB, count: 64)
-}
+}