feat: Limit accumulating canvas buffer

[abelonogov-ld]

Summary

RRWeb Canvas is being updated by draw commands and they all kept in the browser memory. Pr makes SR to reset this Canvas cache by pushing FullSnapshot after 10mb of canvas payloads

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Note

Track accumulated canvas draw size with a configurable limit to force full snapshots; serialize exports and update tests.

• Replay exporter (RRwebGraphQLReplayLogExporter):
  • Add configurable canvasBufferLimit and canvasDrawEntourage (defaults ~10MB, 300 bytes) and track generatingCanvasSize/pushedCanvasSize.
  • Trigger full snapshot when accumulated canvas size exceeds limit, in addition to session/size changes.
  • Update size accounting on incremental and full events; flush size after pushPayload.
  • Ensure single-threaded export with Mutex around export to protect counters.
• Tests:
  • Add test canvas buffer limit validating full snapshot when limit exceeded.
  • Pass buffer config into exporter setup; adjust verifiers to accept expected counts.
  • Minor test cleanups/wording tweaks.

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