Add priority preemption to BuildIngestionService

[jaredpar]

Summary

Fixes priority queue starvation where navigating to the builds menu would queue priority builds that had to wait behind already-running normal builds.

Changes

Priority preemption mechanism:

• When PrioritizeBuild() is called, in-flight non-priority tasks are cancelled via per-task CancellationTokenSource
• Cancelled tasks are reset to pending state (no attempt increment) so they'll be retried
• Worker loop uses Task.WhenAny with a SemaphoreSlim priority signal to detect incoming priority work

Cancellation safety (compile-time guarantees):

• All DB write functions (InsertTestsData, InsertTimelineData, InsertPrInfoData, MarkComplete, MarkFailed, etc.) are static local functions that structurally cannot capture the CancellationToken
• All fetch functions (FetchTestsDataAsync, FetchTimelineDataAsync, FetchPrInfoDataAsync) are static and explicitly receive CancellationToken
• CancellationToken threaded through AzdoClient and HelixClient HTTP calls for proper cooperative cancellation

Testing infrastructure:

• DelegateHandler pattern for HttpMessageHandler injection (no subclassing per test)
• AzdoClient.Create(HttpMessageHandler) and HelixClient.Create(HttpMessageHandler) factories
• MaxParallelism configurable via constructor for deterministic test control
• 3 preemption tests covering tests/timeline/helix task types, each verifying:
  • Priority build completes successfully
  • Preempted builds are reset to pending state

Test results

All 174 tests pass, 0 warnings.