Add integration tests that verify the push client can successfully
send metrics to the receiver and they are stored correctly in SQLite.
Tests:
- TestPushClientToReceiver: Direct HTTP POST verification
- TestPushClientIntegration: Full PushClient with env vars
- TestMultiplePushes: Multiple pushes and filtering (parallel-safe)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
