Add comprehensive unit tests for IRC command handling and message formatting

- Implemented IrcCommandHandlerTests to cover various IRC commands including PING, JOIN, PART, and authentication scenarios.
- Added IrcMessageFormatterTests to validate message formatting for different message types such as text, images, files, and audio.
- Created IrcMessageTests to ensure correct parsing of IRC messages and handling of various command formats.
- Introduced TestHelpers to facilitate testing with mock connections and streams, including a FakeChatService and FakeEncryptionService for simulating chat behavior.

docs/changelog/v0.2.5.md

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+# v0.2.5 - IRC Gateway Fixes & Test Coverage
+
+## Fixes
+
+### IRC Gateway
+
+- **Fixed: IRC JOIN history replay showed encrypted gibberish** — `IrcCommandHandler.HandleJoinAsync` now decrypts channel history before formatting for IRC clients (history was encrypted for SignalR transport but sent raw to IRC)
+- **Fixed: `JoinedChannels` race condition** — `IrcClientConnection.JoinedChannels` replaced with thread-safe methods (`JoinChannel`, `LeaveChannel`, `IsInChannel`, `GetJoinedChannels`) using lock synchronization; prevents crashes when broadcaster threads read while the command handler writes
+- **Fixed: `RequireRegistered` fire-and-forget** — converted from sync `bool` to `async Task<bool>` (`RequireRegisteredAsync`) so the error reply is properly awaited before the handler returns
+
+## Infrastructure
+
+- IRC abstraction layer test suite: IrcMessage parsing, IrcMessageFormatter, IrcClientConnection, IrcCommandHandler, IrcBroadcaster (214 total tests)
+- Test helpers: `TestDuplexStream`, `TestIrcConnectionFactory`, `FakeChatService`, `FakeEncryptionService` for IRC unit testing without network I/O