[path_provider] Re-release FFI foundation version

[stuartmorgan-g]

This re-lands the FFI-based implementation from #9762, which was reverted in #10517.
It contains the following changes (other than version and changelog updates):

• Requires Flutter 3.38.4, to pick up the fix for Running flutter tests on linux might require clang and lld even if the app does not target linux since 3.38 flutter#178715
• Requires objective_c 9.2.1, to pick up the fixes for Failed to get ApplicationDocumentsDirectory: Invalid argument(s): Couldn't resolve native function 'DOBJC_initializeApi' in 'objective_c.framework/ objective_c' flutter#178915 and Asset build fails with "Null check operator used on a null value" in CodeConfig._fromJson dart-lang/native#2824

Also makes necessary repo-level changes:

• Updates the tool's Flutter->Dart version mapping to recognize 3.38.4, to make that minimum work
• Update the release action to use 3.38.5, to avoid having publishing due to using an older version of Flutter than the package requires.

Fixes flutter/flutter#111033

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Footnotes

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Code Review

This pull request significantly refactors path_provider_foundation by migrating its implementation from a Pigeon-based Flutter plugin to direct FFI calls to Foundation, updating the package to version 2.6.0 and requiring Flutter 3.38.4/Dart 3.10. The change involves deleting all Pigeon-generated Swift and Dart files, along with the Swift plugin implementation, and introducing new FFI bindings (ffi_bindings.g.dart), a real FFI-based implementation (path_provider_foundation_real.dart), and a stub for non-FFI platforms. The example project's pubspec.yaml is updated to include ffi, objective_c, mockito, and build_runner for the new FFI and testing setup, and its build.yaml is added for mock generation. Xcode project files for iOS and macOS examples are modified to reflect the new plugin registration and build phases. Integration tests are extensively rewritten to support mocking of FFI calls and platform variants, with tests now split into end-to-end and unit groups, including new tests for macOS-specific path handling. Review comments suggest making several async methods synchronous by wrapping their results in Future.value() for performance and style, and also recommend ensuring consistency for the FakePlatformProvider class across test files for better test reliability and expressiveness.