Explicitly set the JWTGenerator private_key field to None for service logs retrieval (#54442)

In a recent change, the private_key parameter is left empty when instantiating the JWTGenerator used to authenticate log serving requests for in-progress tasks. Such requests use the shared secret_key instead. However, leaving private_key unset means that it is automatically populated through the attrs field factory and this leads to a failure in the post init which asserts mutual exclusivity of secret_key and private_key. The result is stack traces in the API Server logs like the following when retrieving "live" logs:

```
[2025-08-12T19:33:59.403+0000] {file_task_handler.py:907} ERROR - Could not read served logs
Traceback (most recent call last):
  File "/usr/local/lib/python3.12/site-packages/airflow/utils/log/file_task_handler.py", line 878, in _read_from_logs_server
    response = _fetch_logs_from_service(url, rel_path)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/airflow/utils/log/file_task_handler.py", line 173, in _fetch_logs_from_service
    generator = JWTGenerator(
                ^^^^^^^^^^^^^
  File "<attrs generated methods airflow.api_fastapi.auth.tokens.JWTGenerator>", line 34, in __init__
    self.__attrs_post_init__()
  File "/usr/local/lib/python3.12/site-packages/airflow/api_fastapi/auth/tokens.py", line 417, in __attrs_post_init__
    raise ValueError("Exactly one of private_key and secret_key must be specified")
ValueError: Exactly one of private_key and secret_key must be specified
```

The fix is to add back the explicit `private_key=None`, for which we have to add a mypy type ignore comment as it does not seem to understand that `None` is allowed for `AllowedPrivateKeys | None`.

Author: ianbuss

airflow-core/src/airflow/utils/log/file_task_handler.py

@@ -175,6 +175,9 @@ def _fetch_logs_from_service(url: str, log_relative_path: str) -> Response:
         secret_key=get_signing_key("api", "secret_key"),
         # Since we are using a secret key, we need to be explicit about the algorithm here too
         algorithm="HS512",
+        # We must set an empty private key here as otherwise it can be automatically loaded by JWTGenerator
+        # and secret_key and private_key cannot be set together
+        private_key=None,  #  type: ignore[arg-type]
         issuer=None,
         valid_for=conf.getint("webserver", "log_request_clock_grace", fallback=30),
         audience="task-instance-logs",