fix(acp): return invalid params for unsupported session mode

[kyzhang-melo]

Related Issue

None.

Description

This PR improves ACP session mode validation in set_session_mode.

Previously, unsupported mode_id values were rejected with an assert, which could surface as a generic internal error path. This change aligns set_session_mode with the existing ACP handler style used in nearby methods such as set_session_model, by returning structured acp.RequestError.invalid_params(...) errors instead.

Changes:

• return invalid_params when mode_id != "default"

• return invalid_params when session_id does not exist

• add ACP protocol tests for:

  • unsupported session mode
  • unknown session id

Validation:

• uv run pytest tests/acp/test_protocol_v1.py -q
• make format
• make check

Checklist

• I have read the CONTRIBUTING document.
• I have linked the related issue, if any.
• I have added tests that prove my fix is effective or that my feature works.
• I have run make gen-changelog to update the changelog.
• I have run make gen-docs to update the user documentation.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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