fix(review): prevent generated test markdown breakout

[JSONbored]

Motivation

• Close a stored-comment Markdown injection vector where AI-generated Playwright tests containing triple-backtick runs could close the bot's fixed ```typescript fence and inject attacker-controlled Markdown into PR comments.
• Keep public comment rendering safe while preserving the ability to post the generated test source for maintainer review.

Description

• Add markdownFenceFor(source: string) and use a fence length that is one backtick longer than any run found in the generated source so the renderer no longer emits a fixed triple-backtick opener/closer (src/review/e2e-test-gen-render.ts).
• Replace the hard-coded code-fence strings with the computed fence when rendering the AI-generated test body so nested backtick runs cannot break out of the code block (src/review/e2e-test-gen-render.ts).
• Add a regression unit test that includes attacker-controlled backtick fences inside generated Playwright-like source and asserts the renderer uses a longer fence and keeps the content inside the code block (test/unit/e2e-test-gen-render.test.ts).

Testing

• Ran npx vitest run test/unit/e2e-test-gen-render.test.ts and all tests in that file passed.
• Ran npm run typecheck and git diff --check, both succeeded with no errors.
• Attempted npm run test:coverage; the tests passed but coverage remapping failed in this container under Node v24.15.0 while the repo expects Node 22, so a full unsharded coverage run / npm run test:ci should be executed in the project CI or a Node 22 environment before pushing.

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Codex Task

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.95%. Comparing base (c823f9c) to head (0d61629).
⚠️ Report is 10 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4383   +/-   ##
=======================================
  Coverage   93.95%   93.95%           
=======================================
  Files         400      400           
  Lines       36817    36821    +4     
  Branches    13451    13451           
=======================================
+ Hits        34592    34596    +4     
  Misses       1569     1569           
  Partials      656      656           

Files with missing lines	Coverage Δ
src/review/e2e-test-gen-render.ts	100.00% <100.00%> (ø)

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