fix(server): prevent devenv.exe from hanging on shutdown

[CalvinAllen]

Resolves #97

The problem

MCPServerPackage.Dispose runs on the Visual Studio UI thread and blocked it on ServerManager.StopAsync(). That method had no ConfigureAwait(false) anywhere, so every await inside captured the UI thread's synchronization context and posted its continuation back to the thread already blocked waiting for it. Classic deadlock: the main window closes, the UI thread never returns from Dispose, and devenv.exe stays in Task Manager.

The reporter noted this only happens when the server is running, which is the confirming detail. With no child process and no listening pipe, RequestShutdownAsync and both StopAsync methods return already-completed tasks, awaiting those continues inline, and nothing is ever posted back.

The fix

• ConfigureAwait(false) throughout the shutdown path in ServerProcessManager and RpcServer, so no continuation ever needs the caller's thread back.
• Package disposal runs shutdown on Task.Run and waits with a timeout. Task.Run starts with no synchronization context, so nothing can post back to the UI thread; the timeout bounds the damage if anything ever does.
• Bounded cooperative shutdown. A half-open named pipe could leave the RPC shutdown call pending indefinitely — the old 5s + 2s waits had no ceiling in that case. Now: 1s for the RPC ack, 1.5s for a clean exit, then kill with a 0.5s wait.
• Kill-on-close job object. The server process is assigned to a Windows job object with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE, so it dies with Visual Studio even if devenv.exe crashes or is killed from Task Manager. This fixes the mirror-image problem nobody had filed yet: an orphaned server holding port 5050 against the next VS session.
• VSTHRD002 is no longer suppressed project-wide. That analyzer exists to catch exactly this deadlock and was globally disabled. It is now suppressed only at the two Dispose call sites that genuinely need a blocking wait, each with a written justification.
• Two latent bugs in the pipe listener. Cancelling during the retry backoff threw out of a catch block and faulted the listener task; the cancellation source could also be disposed while the listener was still observing its token.

Tests

This adds the repo's first test project (CodingWithCalvin.MCPServer.Tests, xunit on net48) plus a Test workflow, since the existing build workflow only compiles the VSIX project and would not have run them.

• ServerShutdownTests drives the shutdown path from a thread whose synchronization context silently drops posted callbacks — the observable behaviour of a UI thread blocked in Dispose. Both tests were verified to fail against the previous code and pass against this change.
• ProcessJobObjectTests covers the new P/Invoke wrapper, including that closing the job actually terminates an assigned process.

[CalvinAllen]

🔧 CI consolidation

Dropped the standalone test.yml I initially added — it rebuilt the extension and its self-contained server publish a second time on every PR, since the test project references the extension project.

Tests now run from build.yml via a new optional test-project input on the shared workflow: CodingWithCalvin/.github#76.

⚠️ Merge order: that PR must merge first, otherwise build.yml here passes an input the reusable workflow does not yet accept and the run fails.