[various] Update all packages to Pigeon 26 (#10450)
Pigeon versions <26.0.4 aren't compatible with analyzer 8.x, which is causing conflicts for flutter/flutter, so this updates the entire repo to 26.
Related: https://github.com/flutter/flutter/issues/178270
Manual fixes:
- Updates camera_android_camerax and webview_flutter_android for https://github.com/flutter/packages/pull/9515
- Adds the pigeonVar_* overrides to the url_launcher_windows tests
- Removes equality extensions to Pigeon data classes in in_app_purchase_storekit, as they are now generated by Pigeon, causing redeclaration errors.
## Pre-Review Checklist
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[path_provider] Convert iOS/macOS to FFI package (#9762)
Converts path_provider_foundation from a Flutter plugin to a Dart-only package using ffigen to call into Foundation directly.
Most of the Dart unit tests had to be converted to integration tests, although they are still conceptually unit tests (i.e., they still inject mocks for the platform calls). This is because unlike Pigeon, where the API layer being mocked uses Dart types, the FFI calls that we now mock use Obj-C types, and those don't exist outside of an actual app runtime, so the integration test driver is the only place they can be hosted. This is very similar to how most of our web plugin implementations are unit tested.
Fixes https://github.com/flutter/flutter/issues/111033
## Pre-Review Checklist
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[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.