### Description
This PR adds localization support for the **"Back"** and **"Cancel"**
buttons used in `CupertinoNavigationBar`.
#### Changes:
- Introduced `cancelButtonLabel` and `backButtonLabel` in
`CupertinoLocalizations`.
- ~Updated all ARB files with appropriate translations.~
- Modified `_NavigationBarStaticComponents` and `_BackLabel` to use
these labels.
- Ensured all localization tests pass.
- Fixed the test `Auto back/close button` in the
`packages/flutter/test/cupertino/localizations_test.dart`.
### Why is this needed?
Currently, the **"Back"** and **"Cancel"** (prev. **"Close"**) buttons
in `CupertinoNavigationBar` are hardcoded in English. This update
ensures they are properly localized across supported languages.
### Tests
One test failed (even before my changes): `Saturation is applied before
blur`
#### Manual Testing with the `modal_bottom_sheet` Package
I tested the change using [my custom mini
app](https://gist.github.com/Michae1Weiss/9afee8c425d03e3d41e331fdbf21344f)
and the `modal_bottom_sheet` package. I verified that the button is
correctly translated into German localization.
##### Before
##### After
### Button Name Change: From "Close" to "Cancel"
I have renamed the button from **"Close"** to **"Cancel"** to align with
the iOS native naming convention for buttons in modal dialogs.
#### Example:
### Related Issues
[CupertinoNavigationBar's "Close", "Back", and "Cancel" buttons are not
localized. **#48616** ](https://github.com/flutter/flutter/issues/48616)
[CupertinoNavigationBar's "Back" button is not localized **#120722**
](https://github.com/flutter/flutter/issues/120722)
## 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 `///`).
- [ ] 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].
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