fix(flow-chat): keep streaming code preview tail visible

[limityan]

Summary

• Adjust streaming CodePreview tail sizing when nested auto-scroll is disabled so the rendered tail fits the preview viewport.
• Keep overscan for previews that still use autoScrollToBottom and keep completed previews rendering full content.
• Add regression coverage and update the tool-card preview scrolling contract.

Details

CodePreview now derives the streaming tail line limit from the preview height and only includes overscan when the preview is allowed to auto-scroll internally. For compact Write/Edit streaming previews, the latest generated code lines remain inside the nested viewport without requiring per-update scrollTop writes.

The tradeoff is limited to the disabled nested auto-scroll path: compact streaming previews show slightly less tail context while content is still streaming. Completed previews continue to render the full content or full diff.

Verification

• pnpm --dir src/web-ui run test:run -- src/flow_chat/components/CodePreview.test.tsx src/flow_chat/tool-cards/FileOperationToolCard.test.tsx
• pnpm run type-check:web
• pnpm run lint:web
• pnpm run check:repo-hygiene
• git diff --check