test(video_gen): cover gateway decision matrix gaps and 4xx error path

- Add test for 4xx ValueError with actionable remediation message
- Add test for is_available() returning True via managed gateway
- Add test for prefers_gateway overriding direct FAL_KEY
- Add test for is_available() via gateway in plugin test file

tests/plugins/video_gen/test_fal_plugin.py

@@ -105,6 +105,19 @@ def test_fal_generate_requires_fal_key(monkeypatch):
     assert result["error_type"] == "auth_required"
 
 
+def test_fal_available_via_gateway(monkeypatch):
+    from plugins.video_gen.fal import FALVideoGenProvider
+    from plugins.video_gen import fal as fal_plugin
+
+    monkeypatch.delenv("FAL_KEY", raising=False)
+    monkeypatch.setattr(
+        fal_plugin,
+        "_resolve_managed_fal_video_gateway",
+        lambda: object(),  # truthy sentinel — gateway is available
+    )
+    assert FALVideoGenProvider().is_available() is True
+
+
 class TestFamilyRouting:
     """The headline behavior: image_url presence picks the endpoint."""