fix(csharp): rewrite FeatureFlagCacheE2ETests to match production API

Rewrite E2E tests for FeatureFlagCache to use the actual production API:
- Use MergePropertiesWithFeatureFlagsAsync() instead of non-existent
  IsTelemetryEnabledAsync()
- Use HasContext(), TryGetContext(), RemoveContext(), Clear() for
  cache management testing
- Remove tests for non-existent reference counting (RefCount property)
- Remove tests for non-existent TelemetryEnabled, LastFetched, IsExpired
- Fix namespace import from Telemetry to correct namespace
- Add tests for TryGetHost protocol stripping and Uri extraction

The previous tests were testing an API that didn't exist and would not
compile.

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

Author: Jade Wang