ref: add feature flag around BE wxo deployments (#12365)

* Add feature flag around BE wxo deployments

* remove old comment

* [autofix.ci] apply automated fixes

* [autofix.ci] apply automated fixes (attempt 2/3)

* fix: improve consistency in feature flag logging and api
- Use consistent DEBUG log level for disabled-flag path in both
  register_builtin_adapters and register_builtin_deployment_mappers
- Remove dead __getattr__ lazy-load hook and deployment_router from
  __all__ in api/v1/__init__.py (no internal callers remain)
- Change _ensure_deployments_enabled_for_filters from 404 to 400 with
  descriptive error message naming the wxo_deployments feature flag

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Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Hamza Rashid <hzarashid@gmail.com>