fix(ai): contain long markdown code blocks

[openai0229]

Related issue

Closes #1905

Summary

• Remove ReactMarkdown's redundant outer pre wrapper around the custom fenced-code renderer.
• Bound ordinary fenced-code containers to the assistant response width.
• Keep long unbroken SQL and other code lines horizontally scrollable inside the inner pre without widening the chat page.

Affected surfaces

• Frontend / Web
• Backend / API / Storage
• Database plugin / Driver
• JCEF / Desktop packaging
• CI / Build / Release
• Documentation only

Verification

• Commands and results:
  • yarn eslint src/blocks/AI/index.tsx src/blocks/AI/style.ts --max-warnings=0 - passed with zero warnings.
  • yarn build:web:community - production Community Web build passed.
  • git diff --check origin/main...HEAD - passed.
• Manual verification:
  • Used playwright-cli against the Community dev frontend with a mocked SSE response containing one 4705 px SQL line.
  • Checked 1440x900, 1024x768, and 800x700 viewports.
  • At every viewport, document scrollWidth equaled clientWidth, the code block fit the assistant container, and the inner pre retained scrollWidth > clientWidth.
  • Confirmed the assistant response contains one inner pre and no redundant direct outer pre.
• UI evidence:
  • Before: Issue Long SQL code blocks break the AI chat page layout #1905 reproduction shows the long code block expanding the chat layout.
  • After: Playwright measured a 752 px code viewport with 4705 px scrollable content while the 1440 px document remained exactly 1440 px wide.

Risk and compatibility

• Public API or stored data: N/A - frontend rendering and layout only.
• Database or driver compatibility: N/A - no SQL execution or driver behavior changed.
• Network, privacy, or security: N/A - no request, response, or persistence behavior changed.
• Community / Local / Pro boundary: Community frontend only; no Enterprise repository changes are included.
• Backward compatibility: Inline code, SQLPreview, chart fences, copy actions, and ordinary fenced-code rendering remain intact; only the redundant wrapper and overflow sizing change.

Reviewer map

• Start here: chat2db-community-client/src/blocks/AI/index.tsx at renderMarkdown, then src/blocks/AI/style.ts at codeBlockWrap and codeBlockPre.
• Failure condition: A long fenced-code line widens the document or assistant response instead of scrolling inside the code block, or fenced SQL/chart/ordinary code stops rendering.
• Rollback or disable path: Revert commit 78e1beec5dd1c729460c8219cfb1609aa6c2ecf1.

Contributor declaration

• I linked the Issue that defines this change.
• I tested the affected behavior and reported the actual results above.
• I did not include credentials, private data, or generated build output.
• I disclosed substantial AI assistance below, or this PR contains no substantial AI-generated code.

AI assistance: Substantial AI assistance was used for source tracing, implementation, and Playwright verification; the resulting diff was validated with targeted ESLint, a production Community Web build, and measured browser assertions.

[openai0229]

CI baseline note:

The Frontend lint, test, and build job in run 29818404687 stopped before ESLint and the Web build at Validate localization catalogs with the existing two errors:

• es-ES/setting.ts: English source changed; update the translation and its source hash
• ko-KR/setting.ts: English source changed; update the translation and its source hash

Current main@50302ee23f8be3855c5b154c966aad66d575931e failed in the same step with the same two messages in run 29817549352. This PR changes only src/blocks/AI/index.tsx and src/blocks/AI/style.ts; it does not touch localization catalogs or the validator.

The affected files passed targeted ESLint locally, and yarn build:web:community completed successfully. The unrelated localization baseline repair is intentionally not included in this PR.