fix(messaging,macos): fix an issue where getInitialMessage could hang forever in macOS

[Lyokone]

Description

On macOS, plugin registration could occur after the application-finished-launching notification, leaving getInitialMessage() waiting forever because native notification setup never ran. This initializes late registrations immediately, safely extracts macOS launch notification data, and retains the first notification response as the initial message.

Related Issues

• Fixes 🐛 [firebase_messaging] getInitialMessage() does not terminate on macOS if app starts from terminated state #11014

Verification

• flutter test in packages/firebase_messaging/firebase_messaging
• Analyze firebase_messaging_e2e_test.dart
• flutter build macos
• flutter build ios --simulator --no-codesign
• Run the macOS integration test; its host currently fails to build because the Firebase Messaging SPM target cannot find PrivacyInfo.xcprivacy.
• Confirm a terminated-state notification launch on macOS returns the initial message.

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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