fix(elements): keep last good jsx preview when streamed chunk fails to parse

[ephraimduncan]

When a streamed chunk failed to parse, the preview went blank instead of holding the last successful render — the broken chunk was recorded as the "last good" JSX and fed back to the parser. This happened because react-jsx-parser reports parse errors synchronously during render, while the error tracking was reset in a passive effect after commit, so the render-phase error always lost to the post-commit reset. The reset now runs as render-phase derived state (the same pattern JSXPreview already uses to clear its error), so a parse error vetoes the last-good update and the previous preview stays visible until a later chunk renders successfully. This also removes the extra error/re-render cycle each bad chunk triggered through the post-commit setHadError(false).

Added two streaming tests (plain and Strict Mode) driving good → invalid → good chunks; both fail before the fix by rendering an empty preview during the bad chunk. 30/30 pass in Chromium.

[vercel]

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