feat(vsix-build): add optional test-project input - #76

The test project references the extension project, so a standalone test
workflow rebuilt the extension and its self-contained server publish a second
time on every PR. Passing test-project to the shared vsix-build workflow runs
the tests in the same job, reusing the build output.

Requires CodingWithCalvin/.github#76.

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

Package disposal blocked the Visual Studio UI thread on ServerProcessManager
.StopAsync(), whose awaits captured that thread's synchronization context and
posted their continuations straight back to it. The shutdown never completed,
so devenv.exe stayed resident after the main window closed.

The deadlock only reproduced with the server running: with no child process
and no listening pipe, every await in the path completed synchronously and
nothing was ever posted back.

- Use ConfigureAwait(false) throughout the shutdown path in
  ServerProcessManager and RpcServer.
- Run shutdown via Task.Run and wait with a timeout in package disposal, so
  no continuation can need the UI thread and VS exits regardless.
- Bound the cooperative shutdown. A half-open pipe could leave the RPC
  shutdown call pending forever; the previous 5s + 2s waits were also
  unbounded in the worst case.
- Assign the server process to a kill-on-close job object so it dies with
  Visual Studio even when devenv.exe terminates abnormally, rather than
  surviving to hold its HTTP port against the next session.
- Stop suppressing VSTHRD002 project-wide. That analyzer flagged this exact
  deadlock; it is now suppressed only at the two Dispose call sites that
  genuinely require a blocking wait, each with a justification.
- Fix the pipe listener faulting its task when cancelled during the retry
  backoff, and avoid disposing the cancellation source out from under it.

Adds a test project with regression coverage that drives the shutdown path
from a thread whose synchronization context cannot run posted callbacks. Both
new tests fail against the previous code and pass against this change.

* ci: run tests from the build workflow instead of a separate one

The test project references the extension project, so a standalone test
workflow rebuilt the extension and its self-contained server publish a second
time on every PR. Passing test-project to the shared vsix-build workflow runs
the tests in the same job, reusing the build output.

Requires CodingWithCalvin/.github#76.