Perf: Throttle parallel typechecks in Find All References

[xperiandri]

Fixes #20127

Problem

Project.FindFSharpReferencesAsync launched one CancellableTask per document in the project simultaneously via CancellableTask.whenAll, causing an unbounded number of parallel typechecks (one per document) at once during Find All References / Rename.

Fix

• Added CancellableTask.whenAllThrottled maxDegreeOfParallelism in CancellableTasks.fs, using a SemaphoreSlim to cap concurrency while preserving cancellation semantics.
• Applied it in Project.FindFSharpReferencesAsync (WorkspaceExtensions.fs), capping concurrency to max 1 Environment.ProcessorCount.

[T-Gro]

🤖 AI review (@expert-reviewer): no significant issues found. Please verify independently.

Reviewed the throttling implementation for correctness:

• use semaphore combined with return! Task.WhenAll(tasks) correctly awaits within the use scope, avoiding the common SemaphoreSlim use-after-dispose pitfall (this would be a bug with return instead of return!).
• semaphore.WaitAsync(ct) is placed before the try, so Release() runs only when a permit was actually acquired — no risk of over-release, and cancelled waiters correctly skip the release.
• The expensive start ct task runs only after acquiring a permit, so concurrency is genuinely capped.
• max 1 Environment.ProcessorCount guarantees a valid (>= 1) semaphore count.

Cancellation and exception-aggregation semantics match the existing whenAll. LGTM.