fix: message viewer performance degradation on expanded rows during streaming

[weeco]

Summary

• Stabilize subComponent identity in MessageTable using useCallback — prevents React from unmounting/remounting all expanded Monaco editors on every MobX-triggered re-render during gRPC streaming
• Replace Monaco automaticLayout with a single debounced ResizeObserver per editor — eliminates cascading layout recalculations and fixes existing Safari infinite loop
• Memoize JSON.stringify in KowlJsonView to skip redundant serialization on re-renders
• Remove dead code (expandedKeys, toggleRecordExpand) left over from the pre-DataTable migration

Root cause

MessageTable is a MobX observer that re-renders on every message push during gRPC streaming search. The subComponent prop was an inline arrow function, so each re-render created a new function reference. DataTable uses this as a React component type — when the reference changes, React treats it as a different component, unmounting the old tree and mounting a new one for every expanded row. Each cycle destroys and recreates Monaco editor instances, causing 21+ seconds of pure scripting on protobuf-schema topics.

Verified with Playwright

Metric	Before	After
Monaco destructions (10 rapid pushes, 2 expanded editors)	~55	0
Monaco creations	~55	0
Editor stability	Flicker/remount each push	Stable, no flicker