[flutter_tools] Prevent deletion of shared native asset hooks outputs as stale (#191272)

When switching branches or build configurations where native assets
hooks are not used, the build system's stale-output cleanup was deleting
shared hook outputs located in `.dart_tool/hooks_runner/shared/`. This
caused subsequent builds that *do* use native assets hooks to fail
because the hook runner skipped execution (since metadata was
up-to-date) while the actual binary files on disk had been deleted.

This change introduces a helper function `_deleteStaleOutputs` to
preserve files in `.dart_tool/hooks_runner/shared/` during stale output
cleanup in both `FlutterBuildSystem.trackSharedBuildDirectory` and
`_BuildInstance._invokeInternal`.

Fixes https://github.com/flutter/flutter/issues/191020

## Pre-launch Checklist

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there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [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.

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[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]:
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[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