AIP-108: Re-enable Dag Processor coordinator schema-version pinning

### Background In `airflow-core/src/airflow/dag_processing/processor.py` (around lines 582–590), the Dag Processor's `_on_child_started` has the coordinator-pinning block commented out behind a TODO: ```python # TODO(jason810496): Uncomment the coordinator-pinning block below once the # Dag Processor <-> Coordinator interaction pattern is settled and # ``CoordinatorManager.for_dag_file`` is reintroduced (it was removed in # astronomer/airflow#1578, which narrowed AIP-108 scope to task execution only). # When a coordinator handles this file, the pin lets the supervisor's IPC # migrator downgrade outgoing head-shape bodies and upgrade incoming # bodies against the lang-SDK's frozen message schema version. # if (coordinator := get_coordinator_manager().for_dag_file(bundle_name, path)) is not None: # self.lang_sdk_msg_schema_version = coordinator.target_msg_schema_version(msg) self.send_msg(msg, request_id=0) ``` The block was disabled because `CoordinatorManager.for_dag_file` was removed in astronomer/airflow#1578, which narrowed AIP-108's initial scope to task execution only. As a result, the Dag Processor currently cannot pin its outgoing IPC frames to a foreign-runtime (lang-SDK) message schema version, so the supervisor's migrator cannot downgrade outgoing head-shape bodies / upgrade incoming bodies for Dag-parsing exchanges with non-Python lang-SDK runtimes. ### What needs to happen 1. Settle the Dag Processor <-> Coordinator interaction pattern for the parsing path (which file extensions / queues route to which coordinator at parse time, and what `StartupDetails`-equivalent message the parser sends). 2. Reintroduce a resolution entry point on `CoordinatorManager` (e.g. `for_dag_file(bundle_name, path)`) or its successor that returns the coordinator handling a given Dag file. 3. Uncomment / re-wire the pinning block in `_on_child_started`, setting `self.lang_sdk_msg_schema_version` from `coordinator.target_msg_schema_version(msg)` before `send_msg`. 4. Add uni…