fix(discord): add auto-thread fallback path

[sgaofen]

Summary

• make Discord auto-thread creation fall back to a seeded parent-channel message when message.create_thread(...) is rejected
• preserve the existing direct path for the common case, but keep auto-threading alive on channels where only the fallback succeeds
• add focused Discord tests for the fallback-success and double-failure paths

Root Cause

Discord's manual /thread flow already had a fallback path when direct thread creation was rejected, but the automatic mention-triggered thread path only tried message.create_thread(...) once and gave up. That left some channels able to thread the first mention but silently fail later mentions when Discord rejected the direct path.

Addresses #8487.

Testing

• python3 -m py_compile gateway/platforms/discord.py tests/gateway/test_discord_slash_commands.py
• uv run --directory /Users/stephenyu/Documents/hermes-agent-wt-8487 --extra dev pytest -o addopts='' tests/gateway/test_discord_slash_commands.py -q

Platform Tested

• macOS 15.x (Apple Silicon)

Contribution Guide Notes

• Reviewed CONTRIBUTING.md and checked for existing open PRs before submitting this scoped bug fix.
• Ran the targeted verification commands listed above for this PR. I have not claimed a full repo-wide pytest tests/ -q pass unless explicitly noted.

[teknium1]

Merged via #11629 — #11629

Your commit (c57a0052) was cherry-picked with authorship preserved. Closes #8487.

The seed-message-then-create-thread fallback pattern was exactly right — handles the case where Discord rejects direct message.create_thread() on subsequent mentions in a channel (API-side state after the first thread creation), but still lets the manual /thread flow path succeed. Proper reason string (Auto-threaded from mention by <user>) and thorough double-failure test coverage. Nice work.