fix(messaging,macos): unwrap UNNotificationResponse launch payload so getInitialMessage returns it

[Lyokone]

Description

Note

This fix was originally reported and written by @steffenhaak in #17231. All credit for
diagnosing it goes to them. It is reopened here only because fork PRs receive 4 of the
46 CI checks on this repo, so the iOS and macOS jobs that exercise this code never ran
on the original branch.

On macOS, NSApplicationLaunchUserNotificationKey can hold a UNNotificationResponse.
That class declares only two properties:

@interface UNNotificationResponse : NSObject <NSCopying, NSSecureCoding>
@property (readonly, copy) UNNotification *notification;
@property (readonly, copy) NSString *actionIdentifier;

The payload lives at notification.request.content.userInfo, so a UNNotificationResponse
does not respond to userInfo and fell through every branch in
application_onDidFinishLaunchingNotification:, leaving remoteNotification as nil.
getInitialMessage() then resolved with null whenever the app was launched by tapping a
notification.

#18457 fixed the related hang (a nil payload no longer blocks forever), but not the
extraction — so the payload was still being dropped.

This adds an explicit UNNotificationResponse branch ahead of the respondsToSelector:
check, leaving the existing NSDictionary and deprecated NSUserNotification paths
untouched. The original PR replaced those paths rather than extending them, which would
have regressed the cases #18457 deliberately handles.

Verified against the macOS 26.5 SDK at runtime:

Class	responds to userInfo
UNNotificationResponse	NO — hence the dropped payload
UNNotificationContent	YES — the keypath used here

Related Issues

• Fixes [firebase_messaging]: Notification click if app is terminated leads to blank screen on launched app #17230
• Supersedes fix(messaging): macos incorrect parsing of notification response in application_onDidFinishLaunchingNotification #17231

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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[steffenhaak]

Thx 🙏