shiki: add HTML and CSS language support

[InukaWijerathna]

Summary

Registers the HTML and CSS Shiki grammars in shiki.config.mjs so fenced code blocks using these languages are picked up and syntax highlighted, as requested in #844.

Test plan

• Built a highlighter with the updated config and rendered sample html/css snippets through codeToHtml — confirmed correctly colored output for both languages
• npm test — no new failures (3 pre-existing failures on this checkout are unrelated Windows path-separator issues, reproducible on main as well)
• npm run lint — clean (pre-existing unrelated warnings only)

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Project	Deployment	Actions	Updated (UTC)
api-docs-tooling	Ready	Preview	Jul 5, 2026 2:42pm

[cursor]

PR Summary

Low Risk
Config-only addition of two Shiki language bundles with no auth, data, or runtime logic changes.

Overview
Extends the API docs Shiki highlighter so fenced html and css blocks get proper grammar-based highlighting instead of plain text.

shiki.config.mjs imports shiki/langs/html.mjs and shiki/langs/css.mjs and registers both in the existing langs list alongside the other API-doc languages.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.00%. Comparing base (9c5f11f) to head (14512a6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #884   +/-   ##
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  Coverage   85.00%   85.00%           
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  Files         179      179           
  Lines       16453    16457    +4     
  Branches     1491     1492    +1     
=======================================
+ Hits        13986    13990    +4     
  Misses       2457     2457           
  Partials       10       10           

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[avivkeller]

While it's good to add these langs, this doesn't close the mentioned issue, since it doesn't allow users to customize the Shiki config