chore: remove unused extend from PlatformInterface

[Lyokone]

Description

FirebasePluginPlatform was an app-facing plugin base class but still extended PlatformInterface, making implementations unnecessarily fragile. This replaces it with FirebasePlugin as the non-platform-interface base and updates FlutterFire plugin classes to extend FirebasePlugin.

Related Issues

• Fixes 🐛 [firebase_core] FirebasePluginPlatform should not extend PlatformInterface, verifyExtends is never called #8945

Checklist

• 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

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

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