fix(firestore): chunk large Android query snapshots

[ben-milanko]

Description

Android query listeners currently convert an entire Firestore QuerySnapshot into one InternalQuerySnapshot and send it through one Pigeon event-channel envelope. Large result sets therefore require the full native model and a second large encoded buffer to coexist. In the downstream reproduction, 6,112 documents produce a single success envelope larger than 30 MiB and can exhaust the Android heap during StandardMessageCodec.writeValue.

This PR keeps one Firestore query and one snapshot listener, but sends each Android query snapshot as a framed sequence of messages capped at 512 KiB by their actual encoded success-envelope size. The platform interface reassembles the frames before emitting anything to the public query stream, preserving the existing atomic snapshot, document ordering, metadata, and document-change semantics. Other platforms continue to send InternalQuerySnapshot directly and pass through unchanged.

It also closes a listener-registration cancellation race so cancelling during registration cannot leave the Firestore listener active.

Validation includes:

• a deterministic 6,112-document fixture proving the legacy envelope exceeds 30 MiB while every replacement envelope stays within 512 KiB;
• protocol, ordering, malformed-frame, oversized-single-document, empty-snapshot, cancellation-race, and legacy-pass-through tests;
• the aggregate Android :cloud_firestore:testDebugUnitTest and existing :firebase_crashlytics:testDebugUnitTest tasks;
• focused Dart tests and analysis for cloud_firestore_platform_interface;
• a downstream debug and release Android build plus the downstream Firestore cache/store regression suites.

This does not paginate the Firestore query and does not multiply document reads.

Related Issues

• Downstream reproduction and Crashlytics report: https://github.com/Railway-Engineering-Solutions/trax/issues/6322
• There is no corresponding flutter/flutter issue yet.

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