Mark #sizeOf getter as synthetic in FFI struct transformation

[Gurleen-kansray]

The #sizeOf getter generated by _addSizeOfGetter() in pkg/vm/lib/modular/transformations/ffi/definitions.dart is currently not marked as synthetic.

The corresponding #offsetOf getters in the same transformation are already marked with isSynthetic = true. For consistency, the #sizeOf getter should also be marked synthetic.

This issue was identified during GSoC 2026 exploration of the Native Memory Inspector project and suggested by Daco Harkes.

This change adds ..isSynthetic = true to the #sizeOf getter in _addSizeOfGetter().

• I've reviewed the contributor guide and applied the relevant portions to this PR.

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[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/483400 has been updated with the latest commits from this pull request.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/483400 has been updated with the latest commits from this pull request.

[dcharkes]

@Gurleen-kansray see the Gerrit CL for test failures.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/483400 has been updated with the latest commits from this pull request.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/483400 has been updated with the latest commits from this pull request.