tests: internal: log_event_decoder: add comprehensive tests for group marker handling

Add new test cases to verify robust handling of group markers and
corrupted data in the log event decoder:

- decoder_invalid_marker_preserves_group_state: Verifies that group
  state (metadata/attributes) is preserved when invalid group markers
  appear mid-group, preventing data loss from corruption

- Enhanced decoder_corrupted_group_timestamps: Updated to verify
  that invalid markers (negative timestamps not -1/-2) are properly
  skipped while continuing to process valid records

- decoder_group_end_start_sequence: Tests edge cases with GROUP_END
  followed by GROUP_START and verifies correct group state transitions

These tests ensure the decoder maintains data integrity and group
associations even when encountering corrupted or invalid group markers,
validating the improved error handling and state preservation logic.

Signed-off-by: Eduardo Silva <[email protected]>

Author: edsiper