Fix fire-and-forget background task in LocalFeatureFlagsLoader

[haacked]

Summary

• Await the polling task during disposal so in-flight API calls complete before resources are disposed, preventing ObjectDisposedException
• Fix disposal ordering in PostHogClient.DisposeAsync() so the API client remains alive while the polling task winds down
• Mark _pollingTask volatile for cross-thread visibility consistency
• Use GetAwaiter().GetResult() instead of .Wait() in all sync Dispose bridges for cleaner exception stack traces
• Add tests for graceful disposal during in-flight poll, double-dispose, and disposal before polling starts

Follows the same pattern established in AsyncBatchHandler (PR #158).

Fixes #159

Test plan

• Existing tests pass on both net8.0 and netcoreapp3.1
• New CompletesGracefullyDuringInFlightPoll test passes on both frameworks
• New DoesNotDisposeTwice test passes on both frameworks
• New CompletesGracefullyWhenPollingNeverStarted test passes on both frameworks