Wire plugins/video_gen/fal/__init__.py to use the same
_ManagedFalSyncClient pattern that image gen already uses.
Changes:
- Add managed gateway resolution, client caching, and
_submit_fal_video_request() that routes between direct FAL_KEY
and Nous gateway modes
- Update is_available() to return True when either FAL_KEY or the
managed gateway is reachable
- Update generate() to use submit+get handle pattern instead of
fal_client.subscribe() directly
- Fix happy-horse endpoint namespace: fal-ai/ → alibaba/ (matches
the tool-gateway allowlist from fal-video-gen branch)
- Surface actionable error on 4xx gateway rejections
Tests:
- 4 new tests in test_managed_media_gateways.py (gateway routing,
client reuse, direct mode fallback, alibaba namespace)
- Updated existing test_fal_plugin.py fixture to use submit/handle
pattern and patch _resolve_managed_fal_video_gateway for isolation
d04b3c193e