Dashboard test surface: invoke a tool by slug (VENDOR_API)

Fire a real VENDOR_API invocation from the Integrations dashboard.

Verifies the parent connection is owned by the caller's org (404 otherwise), then dispatches invoke_tool_by_slug so the operator can confirm the backend action path end-to-end. Emits an integration_tool_call Tinybird row with mode=vendor_api and reason="ui_slack_test_send". An unresolved/disabled slug surfaces as ToolUnavailableError (400).

The backend result stays raw (the backend path is never narrowed). For the field-picker UI we also compute narrowed here (ENG-631) — what the agent would actually receive — using the shared :func:narrow_result. Effective field map: request.response_field_map → the saved tool's response_field_map → none (narrowed=None).