fix(gateway): quiet Slack missing_scope channel directory fallback

[nanckh]

Summary

• Treat Slack users.conversations missing_scope as an expected limited-scope app condition
• Fall back to session history without recurring gateway warnings
• Add tests for not-ok and SlackApiError-like missing_scope responses

Test Plan

• pytest -q tests/gateway/test_channel_directory.py

[teknium1]

Thanks for the focused Slack fallback fix. Current main still emits the reported warning in gateway/channel_directory.py:242-247, and gateway/run.py:20244-20254 refreshes the directory every five minutes. The proposed handling preserves the existing session-history merge at gateway/channel_directory.py:270-276 while quieting only missing_scope responses.

Current main has since moved the Slack session fallback onto asyncio.to_thread(...) in d23990f527cbd33b528e0e107cfb3449e2f1a3fe; the conflicting change can be salvaged mechanically by retaining those awaits while applying the error classification and tests.

Automated hermes-sweeper review.

[teknium1]

Merged via #70192 — your commit was cherry-picked/reapplied onto current main with your authorship preserved in git history: your missing_scope quieting was cherry-picked.

Thanks for the contribution!