fix(storage,windows): dispatch task events on the platform thread

[SelaseKay]

Description

Windows putData progress was posted from a Firebase C++ SDK worker thread. Flutter requires platform-channel messages on the platform thread; concurrent uploads then native-crashed the process.

This PR:

• Owns the EventSink (the handlers stored a unique_ptr&& member and never took ownership)
• Posts TaskStateListener / completion events onto the platform thread via an HWND_MESSAGE dispatcher (same approach as Firestore Windows)
• Keeps the StorageReference that started each upload alive until the SDK Future completes

The concurrent putData e2e (3×13 overlapping uploads + readback) is expected to go green on Windows after this native change. On the tests-only commit, CI logged the off-thread taskEvent warning and the VM service disappeared as soon as the concurrent test started.

Related Issues

• Fixes [firebase_storage] Windows native crash in upload progress callback, sink ownership and thread dispatch #18664

Test plan

• Windows storage e2e (e2e-storage / windows) passes uploads many small files concurrently and reads them back
• No sent a message from native to Flutter on a non-platform thread logs on plugins.flutter.io/firebase_storage/taskEvent
• Android / iOS / macOS / web storage e2e still pass

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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