feat: improve telemetry coverage in api-client crate
- Add telemetry endpoint to mock server for testing
- Add 3 comprehensive tests for record_telemetry functionality:
* test_record_telemetry_success: Tests successful telemetry recording with multiple events
* test_record_telemetry_empty_events: Tests telemetry recording with empty event list
* test_record_telemetry_with_different_event_types: Tests telemetry recording with different event types
Coverage improvements:
- telemetry.rs: 14.29% → 71.43% (+57.14 percentage points)
- api-client crate overall: 50.61% → 56.51% (+5.9 percentage points)
Tests cover HTTP request construction, header setting, and error handling through retry mechanism.
