Add K2V3 fallback tracking parsers

[mingshanhee]

Summary

• Add opt-in k2_v3_tracking reasoning and tool parsers.
• Record K2V3 parser fallback events without changing default k2_v3 parser behavior.
• Attach reasoning/tool parser fallback events to response meta_info when return_meta_info=True.
• Add coverage for tracking parser parity, event recording, event clearing, and meta-info attachment.
• Exclude build* paths from Python package/wheel discovery.

Testing

• git diff --check origin/prod..HEAD
• python3 -m py_compile on touched Python and test files

Codex: pytest was not run locally because no repo .venv/bin/pytest exists and system Python does not have pytest installed.

[flukeskywalker]

@mingshanhee can you run the tests locally?

[mingshanhee]

Yes, the tests are validated and they are working. Previously, I have also started a RL run with these parsers previously, to add reward penalty for tool and reason fallback events.