Make LSP server-busy backpressure cancellation test deterministic under suite load

[Widthdom]

Summary

LspServerTests.RunAsync_ServerBusyBackpressureRetainsEveryRejectedResponse_Issue4721 is timing-sensitive under full-suite load. The active workspace-symbol request can finish its fixed five-second test delay before the queued cancellation is observed, so the test receives a success result and dereferences a missing error object.

Evidence

• A Release net8.0 full-suite run failed at LspServerTests.cs:1542 after about 41 minutes with NullReferenceException.
• A second Release net8.0 full-suite run with TRX logging reproduced the same test failure after 17 seconds.
• Three isolated Release net8.0 reruns passed in about 0.12 seconds each.
• The full Release net9.0 suite passed with 9,720 tests and 403 skips.
• The test was introduced with the closed partial-result/backpressure work in Support LSP partial results and work-done progress for large symbol responses #4721.

Expected behavior / direction

Use a deterministic synchronization boundary for the active request and cancellation instead of matching two five-second timing windows. The test should prove that cancellation is observed while the request is blocked, independent of full-suite CPU or I/O load.

Scope note

Discovered while validating #4750. It is unrelated to the position-file reader change and is not included in that implementation.

[Widthdom]

Work started on branch fix-issue4785.