test(csharp): add E2E tests for FeatureFlagCache (Phase 7 E2E GATE)

Add comprehensive E2E tests for feature flag fetching from real Databricks
endpoints and validate caching and reference counting behavior:

- FeatureFlagCache_FetchFromRealEndpoint_ReturnsBoolean: Tests real endpoint
- FeatureFlagCache_CachesValue_DoesNotRefetchWithinTTL: Validates caching
- FeatureFlagCache_InvalidHost_ReturnsDefaultFalse: Tests error handling
- FeatureFlagCache_RefCountingWorks_CleanupAfterRelease: Tests ref counting

Additional tests cover:
- Cache expiry and refetch behavior
- Null/empty host handling
- Unknown host behavior
- Multiple hosts with independent ref counts
- Concurrent reference counting thread safety
- False value caching
- Cancellation propagation

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Jade Wang