feat(ml_model_downloader)!: remove firebase_ml_model_downloader package

[SelaseKay]

Description

Removes the deprecated firebase_ml_model_downloader and firebase_ml_model_downloader_platform_interface packages from the repository.

The Firebase ML model download service shuts down on 15 June 2027, and Firebase Apple SDK 13 no longer ships FirebaseMLModelDownloader. This PR deletes the packages, their example, the e2e workflow, and the custom models guide. It also removes every reference from the workspace, the tests app, the nightly workflow and dashboard, the CI dependency lists, the version generation scripts, the bug report template, the README, and the docs.

VERSIONS.md, scripts/versions.json, and the changelogs are left unchanged because they record past releases.

Related Issues

None. This is a package removal for the upcoming breaking release.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

firebase_ml_model_downloader will not be published with the next FlutterFire release. Versions already on pub.dev stay available, but model download stops working when the service shuts down on 15 June 2027.

Move hosted models to Cloud Storage for Firebase and download them with firebase_storage. See Migrate TensorFlow Lite models to Cloud Storage.

Test plan

• flutter pub get resolves at the workspace root, and scripts/validate_workspace.dart reports the workspace is valid.
• all_plugins.yaml builds pass without firebase_ml_model_downloader.
• Analyzer jobs pass.

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