Explain asynchronous causes in the setState() called after dispose() error (#187294)

## Description

The `setState() called after dispose()` assertion currently lists only
*a timer or an animation callback* as the situations that trigger it. In
real apps the most common cause is an **asynchronous gap**: code
`await`s a `Future` (a network request, `Future.delayed`, etc.), the
widget is removed from the tree while the work is in flight (for
example, the route is popped), and the continuation then calls
`setState()` on the now-defunct `State`.

This PR extends the existing `ErrorDescription` and the first
`ErrorHint` to:
- name the asynchronous-gap cause alongside timers / animation
callbacks, and
- point at the canonical fix — guarding `setState()` with a `mounted`
check after an `await` (the pattern enforced by the
`use_build_context_synchronously` lint).

No behavior changes — this only makes the diagnostic more actionable,
especially for beginners (the concern raised in the issue).

**Before**
> … This error can occur when code calls setState() from a timer or an
animation callback.

**After**
> … This error can occur when code calls setState() from a timer, from
an animation callback, or after an asynchronous operation (such as an
awaited network request or other Future) completes once the widget has
already been removed from the tree.

…and the first hint now adds that checking `mounted` is *"especially
important after an `await`."*

I deliberately did **not** embed a documentation URL in the message
(suggested in the issue), since the framework error messages avoid
inline links and the existing `mounted` guidance already states the
remedy.

## Related Issues

Fixes https://github.com/flutter/flutter/issues/177615

## Tests

Added `packages/flutter/test/widgets/framework_test.dart` →
**`setState() after dispose() error explains asynchronous causes`**,
which asserts the thrown `FlutterError` message mentions the
asynchronous cause and the `await` / `mounted` guidance. The existing
`Defunct setState throws exception` test continues to pass; `flutter
analyze` reports no issues and `dart format` reports no changes.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[C++, Objective-C, Java style guides]:
https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[CLA]: https://cla.developers.google.com/