Coordinator: detect ambiguous entrypoints and duplicate dag_ids at import time

### Background Subprocess coordinators can now locate language-SDK artifacts through a Dag bundle instead of an explicit filesystem root (#70805). In the co-located default — no explicit root, no `dag_bundle_name` — the coordinator scans the task's own Dag bundle. For `JavaCoordinator` that makes an existing ambiguity the normal case rather than an edge case. With `main_class` unset, `_JarInfo.find` returns the **first** JAR carrying a `Main-Class` in walk order, and `_calculate_classpath` puts every JAR it found on the classpath: https://github.com/apache/airflow/blob/3a08a3d7792335ac9456cd44c92d8de3ae0d6ec1/task-sdk/src/airflow/sdk/coordinators/java/coordinator.py#L126-L144 So a deployment that ships more than one executable JAR in a bundle gets a non-deterministic entrypoint, and if two of those JARs declare the same `dag_id`, which definition actually runs is decided by directory walk order. Nothing warns the user; the task simply runs whichever one was reached first. The Python path already handles the equivalent case: `_build_duplicate_dag_id_warnings` in `airflow-core/src/airflow/dag_processing/collection.py:327` detects a `dag_id` registered from more than one file and emits a `DagWarning(DUPLICATE_DAG_ID)`. Language-SDK Dags have no equivalent, because parsing happens inside the subprocess and there is no import-stage channel to report into. #70805 documents the gap rather than closing it — it adds a TODO in `_build_execute_task_command` (`task-sdk/src/airflow/sdk/coordinators/java/coordinator.py`) and a note in `airflow-core/docs/authoring-and-scheduling/language-sdks/java.rst` telling users to set `main_class` explicitly: ```python # TODO: Scanning a whole Dag bundle without an explicit main_class lets # _JarInfo.find pick the first executable JAR in walk order, so duplicate # entrypoints across bundles resolve non-deterministically — the same # duplicate-dag_id ambiguity the Python Dag path has. Reject it at the # IMPORT_ERROR stage once AIP-85 exposes a…