`xcom_pull()` ignores `default` parameter when `map_indexes` is not set

### Apache Airflow version main (development) ### What happened and how to reproduce it? **Issue Description** In `RuntimeTaskInstance.xcom_pull()`, when `map_indexes` is not specified (default NOTSET path), the code always appends `None` instead of user-provided `default` value when no XCom is found. Location: `task-sdk/src/airflow/sdk/execution_time/task_runner.py` L420 - Current: `xcoms.append(None)` - Expected: `xcoms.append(default)` The explicit `map_indexes` branch (L419-426) handles `default` correctly. **Steps to reproduce:** ```python result = ti.xcom_pull( task_ids="some_task", key="some_key", default="fallback", ) # Expected: "fallback" # Actual: None ``` ### What you think should happen instead? xcom_pull() should return user-provided default value when no XCom is found, as described in the docstring: ▎ "default (None if not specified) is returned if no matching XComs are found." This also affects PlainXComArg.resolve() which passes default=NOTSET. Due to this bug, None is returned instead of NOTSET, so XComNotFound is never raised for custom XCom keys. ### Operating System N/A (code-level bug) ### Versions of Apache Airflow Providers N/A ### Deployment Other ### Deployment details N/A ### Anything else? This bug happens every time xcom_pull() is called without explicit map_indexes and no matching XCom exists. Already have a fix ready with tests. ### Are you willing to submit PR? - [x] Yes I am willing to submit a PR! ### Code of Conduct - [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)