test(log): add SLogAdapter reliability tests

- Add TestSLogAdapter_MultilineMessages verifying message assembly:
  - Single line with newline logged correctly
  - Split messages across calls are assembled
  - Multiple lines in single callback produce separate records
  - Incomplete lines buffered until newline received
  - Empty lines between content handled
- Add TestSLogAdapter_NilContext verifying graceful nil handling:
  - Falls back to "global" scope without panic
  - Multiple nil context calls work correctly
- Add TestSLogAdapter_LogLevels for FFmpeg-to-slog level mapping
- Add TestSLogAdapter_QuietLevelSilent for AVLogQuiet suppression
- Add TestSLogAdapter_ThreadSafety for concurrent logging
- Add TestSLogAdapter_BasicIntegration for slog buffer output

Author: flexiondotorg