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Fix flutter#160886

From the context of the issue, user wants to observe open/close events
for search view with SearchController.isOpen. However, I think that is
not a friendly approach/solution due to it relies on a
TextEditingController, it is a ValueNotifier which we implicitly
understand is dedicated to text input.

During the investigation, I found that `viewOnClose` works perfectly
because it is a callback from `_SearchViewRoute` (which is a PopupRoute
so it has didPush and didPop callbacks that we can leverage). `onClose`
was called on `didPop`, so we can solve this by creating a similar
`onOpen` then call it on `didPush`. Users then can implement both
callbacks for their needs.

In this PR:

- Propose adding SearchAnchor.viewOnOpen and `SearchAnchor.bar`.onOpen
- Improve documentation of SearchController, so that users will not be
confused with `SearchController.isOpen`

#### Demo (after the fix)


https://github.com/user-attachments/assets/7f30b831-e2d7-4a72-bc0f-35c858e3427b



## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[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
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

---------

Signed-off-by: huycozy <huy@nevercode.io>