fix(firestore): large snapshots do not block frame scheduling

[Lyokone]

Description

Firestore query and document snapshot listeners converted and emitted snapshot payloads on the platform UI thread. This moves the native snapshot listener conversion/emission work onto background queues on Android and iOS, reducing frame blocking when large collection snapshots are delivered.

Related Issues

• Fixes 🐛 [cloud_firestore] Listening to firestore collection can freeze UI thread or crash due to memory leak #4129

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. Local e2e red/green was blocked by device/CocoaPods setup; Android debug build and scoped Cloud Firestore analyzer pass locally.
• I updated/added relevant documentation (doc comments with ///).
• The scoped analyzer (dart analyze packages/cloud_firestore/cloud_firestore packages/cloud_firestore/cloud_firestore_platform_interface) 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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