Fix regex-heavy prompt rendering crash

[roackb2]

Summary

• fix the chat renderer hang on unmatched inline markdown markers in regex-heavy prompts
• refactor parseMessageBlocks into smaller reducer-style helpers
• add regression coverage for regex-heavy prompt content and mixed block parsing

Testing

• yarn vitest run src/tests/unit/tui/prompt-input.test.tsx
• yarn build

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid quadratic array copies in message block parser

This helper now clones the full blocks array every time a line is parsed, which makes parseMessageBlocks O(n²) in the number of lines. In contexts this panel already handles (long assistant outputs, large diffs, or pasted logs), repeated spreads can noticeably increase CPU and memory and reintroduce UI stalls during rendering. The previous implementation used in-place pushes and stayed linear, so this is a regression introduced by the refactor.

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