Fix executor validation error referencing non-existent [core] executors

[roshanprabu]

Summary

_validate_executor_fields (airflow-core/src/airflow/dag_processing/dagbag.py) raises UnknownExecutorException when a task's executor= isn't available, and tells the user to check their "[core] executors configuration". That config key does not exist:

executor_config = conf.get_mandatory_value("core", "executor")

(airflow-core/src/airflow/executors/executor_loader.py) -- Airflow has a single [core] executor key, documented in config.yml as accepting a comma-separated list to configure multiple executors. The plural "executors" only appears in this error message text; grepping the whole tree, it does not exist as an actual config option anywhere.

This is exactly the kind of message a user hits when trying to run a task with an explicit executor= that isn't in their configured list (e.g. from a Helm chart deployment), and the message sends them looking for a setting ([core] executors) that isn't there instead of telling them to make [core] executor a comma-separated list.

Fix: both branches of the message (with and without a team name) now say [core] executor.

Test plan

• Confirmed via conf.get_mandatory_value("core", "executor") that this is the only config key involved -- no plural variant exists in config.yml or anywhere else in the codebase.
• Updated the 4 matching assertions in airflow-core/tests/unit/dag_processing/test_dagbag.py.
• Ran the full test suite for this: pytest tests/unit/dag_processing/test_dagbag.py -k executor -- 12 passed.
• Ran the full test_dagbag.py file (72 tests) to confirm no other breakage -- passes aside from 4 pre-existing errors unrelated to this change (missing airflow_shared module in an unrelated fixture, present before this change too).
• ruff check and ruff format --check pass on both changed files.