fix(mattermost): use per-call aiohttp ClientSession to avoid cross-loop bug

[idarius]

What does this PR do?

Fixes Plugin platform send failed: Timeout context manager should be used inside a task raised whenever an agent invokes send_message (or any cross-channel send) into a Mattermost channel from inside the gateway event loop.

Root cause

MattermostAdapter.connect() cached one aiohttp.ClientSession on self._session, bound to the gateway's main event loop. Every REST helper reused that cached session.

When send_message is dispatched from the agent, model_tools._run_async detects the running gateway loop and spins up a fresh event loop on a worker thread to run the coroutine — see model_tools.py lines 84–143. The cached session is consumed cross-loop, and aiohttp.helpers.TimerContext.__enter__ raises:

RuntimeError: Timeout context manager should be used inside a task

aiohttp's check is asyncio.current_task(loop=self._loop) against the session's loop; the worker-loop bridge has no current task in the gateway loop, so the check fails.

The bug exists since the Mattermost plugin was created; it's been latent because most replies go through the gateway dispatch path (not the send_message tool). It surfaces consistently for cron deliveries and agent-initiated cross-channel sends.

Related Issue

Fixes the runtime described above. No tracking issue was open for Mattermost specifically. Same architectural class as the weixin reports #33445, #31853, #17468, #17267, #18417, #28835, #35237 (Mattermost variant only here).

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• plugins/platforms/mattermost/adapter.py:
  • Add _open_session(timeout_sec) — returns a context manager for a fresh aiohttp.ClientSession bound to the current event loop. Test fixtures that pre-load self._session = MagicMock() are routed through _PassthroughSessionCM so existing tests keep intercepting HTTP calls without modification.
  • Refactor 8 HTTP call sites to use _open_session instead of the cached self._session.<method>(...): _api_get, _api_post, _api_put, _upload_file, _send_url_as_file, the inline downloads in send_multiple_images, and the file-download path in _handle_ws_event.
  • Drop the default ClientTimeout from self._session = aiohttp.ClientSession() in connect() — the cached session is now used only for the WebSocket, which uses heartbeat= for liveness.

+124 / -79 lines, no public-API changes, no new dependencies.

Same architectural pattern as #1736 (fix(gateway): SMS session-per-send, merged) and #3818 (fix(whatsapp): reuse persistent aiohttp session across requests, merged).

How to Test

1. Set up a Mattermost server with a bot account, configure MATTERMOST_URL and MATTERMOST_TOKEN.
2. Start the gateway: hermes gateway start.
3. In any monitored channel, ask the agent to post into another channel via the send_message tool. Example:

   @hermes please send "test" to mattermost:<another-channel-id>
   

4. Before the fix: the gateway logs WARNING agent.tool_executor: Tool send_message returned error … "Plugin platform send failed: Timeout context manager should be used inside a task" and the agent never delivers.
5. After the fix: the message is posted, the tool returns {"success": true, "message_id": "..."}.

Out-of-process and gateway-loop paths still work unchanged:

• hermes send -t mattermost "test" (out-of-process) — unchanged.
• File downloads triggered from incoming Mattermost posts (gateway-loop path) — unchanged.

cd hermes-agent
./venv/bin/python -m pytest tests/gateway/test_mattermost.py -q
# 43 passed

	Before	After
send_message tool from agent context	Plugin platform send failed: Timeout context manager… (every call)	success
_handle_ws_event file download (gateway loop)	success	success (unchanged behaviour)
hermes send -t mattermost "…" out-of-process	success	success (unchanged behaviour)
Mattermost adapter tests	43 pass	43 pass (no test changes)

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate — none for Mattermost
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/gateway/test_mattermost.py -q and all tests pass
• I've added tests for my changes — N/A: existing tests cover the refactored code paths, no behavioural surface added
• I've tested on my platform: Ubuntu 24.04 / Python 3.11 in the bundled venv

Documentation & Housekeeping

• I've updated relevant documentation — N/A (internal helper, no public-facing change)
• I've updated cli-config.yaml.example if I added/changed config keys — N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — N/A (pure asyncio, no filesystem or shell)
• I've updated tool descriptions/schemas if I changed tool behavior — N/A