[BAG] Implement fixture creation (extract fixture)

[zhexuany]

Summary

The robocodec extract fixture command does not work for BAG format files.

Current Behavior

$ robocodec extract fixture input.bag -o /tmp -n test
Error: Fixture creation requires format-specific message iteration. This feature is not yet implemented. Use the convert command for full file copying.

Expected Behavior

Should create minimal fixture files with one message per topic for testing purposes.

Use Case

• Create small test fixtures from large bag files
• Generate test data for CI/CD pipelines
• Create reproducible test cases with real message data

MCAP Status

This feature works for MCAP files, only BAG format is affected.

Related

• Issue [BAG] Implement partial message extraction (extract messages --count) #30 (partial message extraction)
• Issue [BAG] Implement topic-specific extraction (extract topics) #31 (topic-specific extraction)
• Part of BAG feature parity with MCAP