fix(database,windows): marshal listener callbacks onto the platform thread

[SelaseKay]

Description

On Windows, Realtime Database listener callbacks and Future::OnCompletion replies were sent to Flutter from the C++ SDK worker thread. That trips the engine's non-platform-thread check and can crash the process; cancelling a subscription could also delete a listener while OnValueChanged was still running.

This hops EventSink and Pigeon replies onto the platform thread (same HWND dispatcher pattern as cloud_firestore) and serializes listener teardown with in-flight SDK callbacks.

Related Issues

• Fixes [firebase_database, windows] Listener callbacks are delivered on a non-platform thread and crash the process #18630

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.

Test plan

• Windows: subscribe to onValue / child events and confirm no non-platform thread engine error
• Windows: rapid subscribe/cancel loop from [firebase_database, windows] Listener callbacks are delivered on a non-platform thread and crash the process #18630 no longer crashes
• Windows: set / get / runTransaction still complete successfully (OnCompletion replies)
• Existing database e2e (cancels in-flight value listeners on distinct paths without error)

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