fix(im): deliver artifacts/presented files with real filename extension

[xfgong]

Summary

Fixes #511. Feishu (and every send_file connector) derives the delivered file's identity and inline preview from the file_name we send. The artifact display name is often a human title with no extension, so it was sent as e.g. Quarterly Report and Feishu could not open the preview.

• save_artifact (_try_deliver): prefer the real file basename from entry_file/path; fall back to the display name only when that basename has no extension.
• present_file (_try_deliver_presented): derive a missing extension from mime_type via mimetypes.guess_extension.

The broken filename was set in the platform-agnostic IMArtifactDispatcher, so this also hardens Slack/Discord/Teams/DingTalk.

Test plan

• Added regression tests in test_im_outbound_files.py and test_im_outbound_presented.py:
  • display name without extension → uses entry_file filename
  • entry_file takes priority over a display name that already has an extension
  • present_file with extensionless filename → extension filled from mime_type
  • already-extended filename → not double-appended
• uv run pytest tests/integration/test_im_outbound_files.py tests/integration/test_im_outbound_presented.py → 22 passed
• ruff check + ruff format --check clean on changed files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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