fix(discord): keep free-response channels inline

[simpolism]

What does this PR do?

Fix a code-vs-docs gap in Discord auto-threading. Free-response channels are documented as "skip auto-threading — reply inline," but the production code only checks DISCORD_NO_THREAD_CHANNELS when deciding whether to skip thread creation, not DISCORD_FREE_RESPONSE_CHANNELS. So every message in a free-response channel currently spawns its own thread, turning a documented "lightweight chat" channel into a thread-spawning machine.

One-line production change: OR is_free_channel into the existing skip_thread calculation.

The existing docstring for discord.free_response_channels at website/docs/user-guide/messaging/discord.md:347 already promises this behavior:

"Free-response channels also skip auto-threading — the bot replies inline rather than spinning off a new thread per message. This keeps the channel usable as a lightweight chat surface."

so no docs change is needed — this PR makes the code match the docs.

Related Issue

Fixes #25310

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• gateway/platforms/discord.py: in _handle_message's auto-thread block, skip_thread = bool(channel_ids & no_thread_channels) becomes bool(channel_ids & no_thread_channels) or is_free_channel. The is_free_channel variable is already computed earlier in the function (used by the mention gate) — this propagates it to the auto-thread gate as well.
• tests/gateway/test_discord_free_response.py: new regression test test_discord_free_response_channel_skips_auto_thread that mocks _auto_create_thread and asserts it is not awaited when a message arrives in a free-response channel with auto_thread=true (the default).

How to Test

To verify the bug exists without the fix:

1. Check out main.
2. Apply only the test addition from this PR.
3. Run pytest tests/gateway/test_discord_free_response.py::test_discord_free_response_channel_skips_auto_thread.
4. Expect: failure with AssertionError: Expected mock to not have been awaited. Awaited 1 times. (i.e. the production bug is real — auto-thread is fired).

To verify the fix:

1. Apply this full PR.
2. Run the same test — passes.
3. Run the full tests/gateway/test_discord_free_response.py suite — 22 tests pass (was 21).

Manual verification: configure a Discord bot with discord.free_response_channels: [<channel-id>] and discord.auto_thread: true. Send any message in that channel. Before this PR: bot spawns a thread. After this PR: bot replies inline.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits
• I searched for existing PRs — no duplicate
• My PR contains only changes related to this fix
• I've run the impacted test files and all tests pass
• I've added tests — regression test added that demonstrably fails without the fix
• I've tested on my platform: Ubuntu 24.04

Documentation & Housekeeping

• I've updated relevant documentation — N/A; existing docs already describe the intended behavior, this PR makes code match docs
• cli-config.yaml.example — N/A, no config key changes
• CONTRIBUTING.md / AGENTS.md — N/A, no architecture/workflow changes
• Cross-platform impact — N/A, purely server-side adapter logic
• Tool descriptions/schemas — N/A

[teknium1]

Merged via PR #25444 (commit d557544 on main) with your authorship preserved via cherry-pick + rebase merge. Thanks for the clean fix + regression test!