Complete the unregister before shutting the transport down

[calebtt]

Summary

Closes #16. Stacked on #13's branch; merge after it. It uses #13's FakeRegistrar and its _isShutdown check in ScheduleReconnection.

SipClient.Shutdown() called SIPRegistrationUserAgent.Stop(), which queues the unregister (REGISTER with Expires 0) on a thread-pool thread, and then shut the transport down straight away. The unregister is usually challenged (401, then an authenticated retry), so it rarely completed. The registrar kept a dead contact until it expired and forked calls to it. With sipbot serve and quit, a lab extension ended up listing three contacts.

Changes

• Shutdown() now sends the unregister and waits up to 3 s (SipClient.UnregisterTimeout) for the registrar to confirm it (RegistrationRemoved, or a failure) before it shuts the transport down.
• When the client is not registered, it stops the agent and sends nothing, with no wait.
• No reconnection is scheduled once shutdown has begun. This relies on Registration does not recover after an outage: reconnect throws "already running" and IsRegistered stays true #13's _isShutdown check in ScheduleReconnection; the test below covers it.

Tests

New ShutdownUnregisterTests, against #13's scripted loopback registrar:

• A registered client's last REGISTER has Expires 0, both unchallenged and after a 401 challenge. Shutdown returns in under 2 s, and no REGISTER follows (no reconnect after RegistrationRemoved).
• An unregister the registrar never answers is bounded to the 3 s timeout.
• A client that is not registered sends nothing and returns immediately.

Without the fix, the first three fail: the unregister never goes out. dotnet test: 90 passed (86 on #13 + 4 new). Build warnings unchanged (17).

Compatibility

Dispose() and Shutdown() can now take up to 3 s longer when the registrar does not answer the unregister. When it answers, the delay is a round trip or two. SipBotOpen, homeline, and sipbot (quit) now leave no stale contact behind.

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