Follow-up to PR #2504. The original fix covered the two direct FAL_KEY
checks in image_generation_tool but left four other call sites intact,
including the managed-gateway gate where a whitespace-only FAL_KEY
falsely claimed 'user has direct FAL' and *skipped* the Nous managed
gateway fallback entirely.
Introduce fal_key_is_configured() in tools/tool_backend_helpers.py as a
single source of truth (consults os.environ, falls back to .env for
CLI-setup paths) and route every FAL_KEY presence check through it:
- tools/image_generation_tool.py : _resolve_managed_fal_gateway,
image_generate_tool's upfront check, check_fal_api_key
- hermes_cli/nous_subscription.py : direct_fal detection, selected
toolset gating, tools_ready map
- hermes_cli/tools_config.py : image_gen needs-setup check
Verified by extending tests/tools/test_image_generation_env.py and by
E2E exercising whitespace + managed-gateway composition directly.
2e722ee29a