Add CancellationToken and async hydration to GitStatusCache

[tyrielv]

• Add CancellationToken parameter to CreateSummary (default: none)
• Check cancellation between each phase of summary computation
• Separate OperationCanceledException catch returns invalid summary
• GitStatusCache creates CancellationTokenSource, cancels on Shutdown
• Run UpdateHydrationSummary in parallel with TryRebuildStatusCache via Task.Run — independent operations should not delay each other

[KeithIsSleeping]

see below

[KeithIsSleeping]

(Corrected re-post — the original review had rendering issues with backtick escaping)

Clean cancellation support and a good parallelization win. Two suggestions:

1. Race condition on activeHydrationTask — The field is written on the background thread (this.activeHydrationTask = hydrationTask in RebuildStatusCacheIfNeeded) and read in Dispose() on the shutdown path. There's no synchronization between these. If Dispose is called between Task.Run() and the assignment, or if two rebuilds overlap, the stored reference may be stale. Consider Interlocked.Exchange or protecting with a lock.

2. Dispose() waits with no timeout — this.activeHydrationTask.Wait() in Dispose could block the main shutdown path indefinitely if the hydration summary hangs (e.g., waiting on a git process). Consider adding a bounded wait: this.activeHydrationTask.Wait(TimeSpan.FromSeconds(5)).

The cancellation-vs-error distinction is well handled — returning an invalid summary with no Error lets the caller log at Info level instead of Warning, and correctly avoids triggering the circuit breaker from PR #1913. The parallel execution makes sense since hydration summary and git status are independent operations.

Minor: a couple of extra blank lines around cancellationToken.ThrowIfCancellationRequested() calls — cleanup nit.