fix: replace asyncio.sleep FAF guards with deterministic awaits

[ajbozarth]

Misc PR

Type of PR

• Bug Fix

Description

• Link to Issue: Fixes Provide a deterministic way to await FIRE_AND_FORGET hook completion in tests #691

Bumps cpex to 0.1.0.dev12, which adds PluginResult.background_tasks and wait_for_background_tasks() (upstream contextforge-org/contextforge-plugins-framework#33). Uses these to replace all racy asyncio.sleep() guards that were used to wait for FIRE_AND_FORGET plugin completion in tests.

Changes:

• mellea/plugins/manager.py: add drain_background_tasks() — accumulates PluginResult objects with background tasks in invoke_hook(), drains via wait_for_background_tasks() for tests where the hook fires inside a backend call
• test/plugins/test_execution_modes.py: capture invoke_hook result directly and call result.wait_for_background_tasks() (2 tests)
• test/telemetry/test_metrics_backend.py: replace 7× asyncio.sleep(0.05) with drain_background_tasks()

Testing

• Tests added to the respective file if code was changed
• New code has 100% coverage if code as added
• Ensure existing tests and github automation passes (a maintainer will kick off the github automation when the rest of the PR is populated)

Attribution

• AI coding assistants used: Claude Code

[ajbozarth]

Follow-up to address a test failure in test_ollama_token_metrics_integration[non-streaming].

The _pending_background_results global accumulates PluginResult objects across the entire test session. pytest-asyncio creates a new event loop per test, so results from earlier tests hold futures bound to dead loops. When a later test calls drain_background_tasks(), awaiting those stale futures raises RuntimeError: Future attached to a different loop.

Fix: added discard_background_tasks() to manager.py — same as drain_background_tasks but discards without awaiting, safe to call across loop boundaries. The enable_metrics fixture in test_metrics_backend.py now calls it before and after each test to ensure only results from the current event loop are ever drained.

Also clears _pending_background_results in shutdown_plugins() for completeness.

[ajbozarth]

Claude decided to delete my worktree for this PR instead of #895 when I went to clean up and I didn't noticed it picked the wrong branch name

[ajbozarth]

cc @araujof as the original plugins/hook author