Provide feedback when accessing undeclared outlet at runtime

### Description

Currently, operations toward a non-declared outlet to `outlet_events` is simply ignored:

```python
@task
def emit_alias_event():
context = get_current_context()
context["outlet_events"][AssetAlias("a")].add(Asset("new"))
# Line above does nothing since AssetAlias("a") is not an outlet.
```

The same also applies to `yield Metadata`.

This is a bit confusing to the user since they did something, and reasonably expect it’s an Airflow bug if nothing seems to happen.

I can think of a couple of ways to resolve this:

#### Automatically add outlets when you do this

If you access something, it automatically becomes an outlet. However, Airflow would not be able to detect such a dynamic outlet in many other places, such as the dag dependency graph (which is generated statically). Would this only shift the confusion elsewhere?

#### Raise a KeyError

Or a more detailed error with a hint. More explicit, but would it be awkward for certain workflows?

### Use case/motivation

_No response_

### Related issues

_No response_

### Are you willing to submit a 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 runtime handling of context["outlet_events"] and yielded Metadata in the task example. Decide between automatic outlet registration and raising a KeyError or more detailed error, then define the expected behavior for undeclared outlets and verify it in the relevant runtime tests.

Written by the indexing model from the issue text.