infra: ensure markdown rendered is only ever initialized once

[ST-DDT]

Implement suggestion from shikijs/shiki#1289

• Random CI failures due to light theme color differences shikijs/shiki#1289
• https://github.com/faker-js/faker/actions/runs/27549969244/job/81433308467

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Store the promise to ensure we only ever initialize the markdown rendered backend once.
This is an attempt to fix the flaky markdown rendered test.

While I don't think that it is the cause for the failure, it does fix the unintended side effects of calling initMarkdownRenderer() multiple times.

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Pull request overview

Caches the Markdown renderer’s initialization promise to prevent concurrent duplicate initialization.

Changes:

• Adds a shared renderer promise.
• Reuses the in-flight promise across initialization calls.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.92%. Comparing base (d4b35a4) to head (82b79c3).

Additional details and impacted files

@@           Coverage Diff           @@
##             next    #3984   +/-   ##
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  Coverage   98.92%   98.92%           
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  Files         926      926           
  Lines        3241     3241           
  Branches      588      568   -20     
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  Hits         3206     3206           
  Misses         31       31           
  Partials        4        4           

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