fix: event handler exception safety + non-happy-path tests

[Kiryuumaru]

Summary

Fixes event handler exception safety and adds comprehensive non-happy-path test coverage.

Bug Fixes

Event handler exceptions crash the mux (fixed)

All event invocations (ChannelOpened, ChannelClosed, ChannelAccepted, Connected, Ready, Disconnected, Error, Reconnecting) are now wrapped in RaiseEvent helper methods with try-catch. A throwing user event handler can no longer crash the multiplexer, channels, or dispose.

Duplicate channel ID silently broken (fixed)

ChannelRegistry.RegisterPendingAcceptChannel used indexer assignment which silently overwrites. Changed to TryRegisterPendingAcceptChannel with TryAdd — second OpenChannel("same") now correctly throws MultiplexerException(ErrorCode.ChannelExists).

New Tests (92)

UnhappyPathTests.cs (53 tests)

• Multiplexer state violations (before start, after dispose)
• Channel operations after close (write, read, double close)
• Cancellation token propagation
• Connection failure scenarios
• GoAway protocol edge cases
• Channel state transitions
• AsStream error paths
• Transit error paths (MessageTransit, StreamTransit, DuplexStreamTransit)
• Event firing on error paths
• Lookup methods with invalid input
• Multiple dispose safety
• Empty and boundary writes
• Stats consistency after errors
• Configuration edge cases

ApiMisuseTests.cs (39 tests)

• Event handler resilience (ChannelOpened, ChannelClosed, Disconnected, Error)
• DuplexStreamTransit misuse (read/write after dispose, null args, CanRead/CanWrite, Seek, multiple dispose)
• DeltaTransit misuse (send/receive after dispose, wrong direction, POCO without TypeInfo, empty batch)
• Concurrent misuse (multiple threads disposing, same-ID race, concurrent write+close, concurrent GoAway)
• GoAway + channel interactions
• Write before ready
• AcceptChannel/OpenChannel misuse (same ID, empty string, very long IDs)
• StreamTransit sync path errors
• Stats verification
• Lookup methods with invalid input
• AsStream after channel close

Test Results

393 total tests, 392 pass consistently (1 pre-existing flaky stress test: ExtremeStressTests.Scale_ConcurrentBidirectional_StressTest).