revert(ai): remove long markdown code fix

[openai0229]

Related issue

Reverts #1907

Related to #1905

Summary

The long Markdown/SQL layout work did not resolve the packaged Community desktop behavior after multiple revisions. At maintainer direction, stop this implementation path and restore the AI Markdown renderer and styles to their state before PR #1907.

This revert removes every change introduced by merge commit 1cf3c535c6f2155067295e9e187e06f17b3bcc0d:

• Restore the original MarkdownCodeBlock language detection and ReactMarkdown pre behavior.
• Remove the width constraints added to the page-owned code shell.
• Delete the markdownCode helper and its focused test.
• Remove only that test's package script and CI step while preserving all later unrelated test entries.

The unmerged follow-up PR #1960 was closed separately and its branch/artifacts were deleted.

Affected surfaces

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

Verification

• Commands and results:
  • git revert -m 1 1cf3c535c6f2155067295e9e187e06f17b3bcc0d - completed; conflicts were limited to later adjacent CI/package entries.
  • yarn eslint src/blocks/AI/index.tsx src/blocks/AI/style.ts --max-warnings=0 - passed.
  • CI workflow YAML parse - passed.
  • package.json JSON parse - passed.
  • git diff HEAD^ --check - passed.
  • yarn build:web:community --app_version=5.3.1 - passed; Webpack compiled successfully.
• Scope verification:
  • The revert changes exactly the six files changed by PR fix(ai): contain long markdown code blocks #1907 and reverses its +86/-4 patch as +4/-86.
  • src/blocks/AI/index.tsx and src/blocks/AI/style.ts match the pre-merge blobs from 975592ce.
  • markdownCode.tsx, markdownCode.test.tsx, and all associated references are absent.
  • Later unrelated active-tab-locator, execution-console, and Community operations entries remain present.
• UI evidence: N/A. This intentionally restores the pre-fix(ai): contain long markdown code blocks #1907 rendering behavior, including the unresolved long-code layout limitation tracked by Long SQL code blocks break the AI chat page layout #1905.

Risk and compatibility

• Public API or stored data: N/A; frontend rendering only.
• Database or driver compatibility: N/A.
• Network, privacy, or security: N/A.
• Community / Local / Pro boundary: Community frontend only.
• Backward compatibility: This deliberately reintroduces the pre-fix(ai): contain long markdown code blocks #1907 Markdown rendering and long-line layout behavior. No persisted data or migration is involved.

Reviewer map

• Start here: compare this commit with merge commit 1cf3c535, then inspect the preserved neighboring entries in .github/workflows/ci.yml and chat2db-community-client/package.json.
• Failure condition: Any code from PR fix(ai): contain long markdown code blocks #1907 remains, or any later unrelated test/CI behavior is removed.
• Rollback or disable path: Revert this revert to restore PR fix(ai): contain long markdown code blocks #1907; no migration or cleanup is required.

Contributor declaration

• I linked the Issue and PR that define this rollback.
• I tested the affected build 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: Codex identified the exact merge commit, performed the first-parent Git revert, preserved later unrelated conflict-side additions, verified blob and patch equivalence, and ran the reported checks. The maintainer explicitly directed abandonment and rollback of this fix path.