refactor(firestore,apple): migrate iOS/macOS plugin implementation to Swift

[SelaseKay]

Description

Migrates the cloud_firestore Apple plugin from Objective-C to Swift for both iOS and macOS, following the same pattern as packages like firebase_auth, firebase_core, and firebase_crashlytics.

• Replaces the ObjC implementation with Swift while keeping the FLTFirebaseFirestorePlugin class name
• Switches Pigeon codegen from objcOut to swiftOut (FirestoreMessages.g.swift)
• Lowers pipeline expressions onto Firebase's typed Swift Expression APIs (Field, Constant, DocumentMatches, Expression.equal(), and so on)
• Keeps a small ObjC++ shim for SetClientLanguage, which is not callable from Swift
• Updates CocoaPods and Swift Package Manager packaging
• macOS sources continue to symlink to the shared iOS implementation

Dart, Android, and Windows APIs are unchanged. Plugin users do not need to update their apps.

The cloud_firestore iOS pod target was compiled successfully. macOS and Firestore integration tests were not run in this environment. Android Kotlin migration is not part of this PR.

Related Issues

None

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.

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[wiz-9635d3485b]

Wiz Scan Summary

Scanner	Findings
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