test: add endpoint coverage for bridge_adapter plumbing

Covers three paths that shipped without regression coverage:
- GET /agents/{id}/bridge-status response shape (native vs openclaw,
  connected vs disconnected, optional-field fallbacks)
- PATCH /agents/{id} silent-drop guard for native agents
- POST /agents/ default bridge_adapter resolution (claude_code for
  openclaw, NULL for native even if client sends a value)

Author: TatsuyaGO