Inconsistency between yielding Metadata and outlet_events extras for partitioned Assets

### Under which category would you file this issue?

Airflow Core

### Apache Airflow version

Airflow 3.2.1

### What happened and how to reproduce it?

I have a DAG that produces partitioned Assets. It contains a dynamicaly mapped task group of which one task has an Asset defined an outlet and emitts _AssetEvents_ for each expanded instance of the TaskGroup.

When I used `yield Metadata` the downstream DAG was triggered by each _AssetEvent_, so one run of the source DAG could result in e.g. three _DagRuns_ of the downstream DAG.

However, when I upgraded from Airflow 2.10.4 to Airflow 3.2.1, I also switched to attaching extra information to _AssetEvents_ with `outlet_events[ASSET].extra`.

The first issue I encountered was that _AssetEvents_ did not show up in the UI, when `outlet_events` wasn`t a keyword only argument in the producing task function. E.g.
```
def producing_function(input: str, outlet_events=None):
"""Example producing task"""
`outlet_events[ASSET].extra = {"section": "XY"}
```
Did not show AssetEvents. But it triggered the downstream DAG that then hang because the AssetEvent was not accessible.
Making _outlet_events_ a keyword argument like this:
```
def producing_function(input: str, *, outlet_events=None):
"""Example producing task"""
`outlet_events[ASSET].extra = {"section": "XY"}
```
produced visible _AssetEvents_. The downstream DAG gets triggered only once but shows all three _AssetEvents_ with the attached extra metadata from the three mapped taskgroup instances in the producing DAG.

When I use `yield Metadata(ASSET, {"section": "XY"})` in the producing task, each _TaskInstance_ triggers a DagRun of the downstream DAG with the extra metadata properly attached (which I expected to happen and how it was before).

The documentation indicates that `yield Metadata` and `outlet_events[ASSET].extra` are equivalent:
https://airflow.apache.org/docs/apache-airflow/3.2.2/authoring-and-scheduling/assets.html#attaching-extra-information-to-an-emitting-asset-event
For partitioned assets, they clearly are not.

### What you think should happen instead?

The difference in behavior between `yield Metadata(ASSET, {"section": "XY"})` and `outlet_events[ASSET].extra = {"section": "XY"}` - at least for partitioned DAGs - should be clearly documented and outlet_events should be properly produced also when outlet_events are not accessed as keyword only arguments.

### Operating System

Ubuntu 24.04

### Deployment

Docker-Compose

### Apache Airflow Provider(s)

_No response_

### Versions of Apache Airflow Providers

_No response_

### Official Helm Chart version

Not Applicable

### Kubernetes Version

_No response_

### Helm Chart configuration

_No response_

### Docker Image customizations

Not Applicable

### Anything else?

_No response_

### 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)

Start with the linked Airflow assets documentation section on attaching extra information to emitting asset events, then reproduce the mapped partitioned-asset behavior using both Metadata and outlet_events[ASSET].extra. Compare event visibility, downstream DAG runs, and attached metadata; done means the behavior is consistent or the documented difference and keyword-only requirement are explicit.

Written by the indexing model from the issue text.