Block error persists after the failing block is edited or replaced

[xHayden]

Once a block throws, BlockErrorBoundary keeps showing the error for that node id, even after the agent fixes or replaces the block. The message even picks up the new type, e.g. "This sequence block could not be rendered: Cannot read properties of undefined (reading 'from')", even though the error came from the old flow diagram.

Repro (Whiteboard Desktop 0.1.1, via MCP)

1. Trigger any block render error, e.g. the flow diagram edge-removal crash in Flow diagram throws Cannot read properties of undefined (reading 'from') after an edge is removed #608.
2. session_edit {"type":"replace","targetId":"<same block id>","content":{"type":"sequence","title":"Repro sequence","actors":{"a":"A","b":"B"},"steps":[{"from":"a","to":"b","label":"call","explanation":"A calls B."}]}}
3. The block still shows the old error.

Cause

BlockErrorBoundary sets error in getDerivedStateFromError and never clears it (blocks.tsx#L282-L309). DocumentNode is keyed by node.id (api-document.tsx#L343-L344) and replace keeps the id, so the same boundary instance survives the edit (api-document.tsx#L408-L419).

ReviewDocumentBoundary already handles this by resetting when revision changes (review-document-boundary.tsx#L36-L44).

Possible fix

Give BlockErrorBoundary the same reset, keyed on the block (e.g. pass block or a content revision and clear error in getDerivedStateFromProps when it changes), or key the boundary on the block's type + revision.