`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)