fix(mdx-variable-regex): user variable display in SuperHub read mode

[HugoHSun]

• before: {user. matches user + any character
• after: {user\. matches user + literal dot
• add test cases

🚥 Resolves CX-2232

🧰 Changes

The Bug:
The regex /\{user.(...)/ had an unescaped . after user, which matches any character instead of a literal dot. This caused patterns like {userEndpoint} to incorrectly match as a ReadMe variable, where the . matched E and captured ndpoint.

Symptoms:

• ${userEndpoint} rendered as $NDPOINT
• ${userXYZ} rendered as $YZ
• Only visible in SuperHub View mode (syntax highlighter runs client-side with tokenizeVariables: true)

The Fix:
Escape the dot to match only a literal . character:

• Before: {user. → matches user + any character
• After: {user\. → matches user + literal dot only

Added test cases to ensure the regex does not match JS template literal patterns without an actual dot after user.

🧬 QA & Testing

1. Create a doc with a code block containing JS template literals:
   const url = ${baseUrl}/${userEndpoint}/login;
   const ok = ${userXYZ};

2. Save and view the page (both Edit preview and public View)

3. Expected: Code renders verbatim - ${userEndpoint} and ${userXYZ} appear exactly as written

4. Before fix: Variables were mangled - ${userEndpoint} → $NDPOINT, ${userXYZ} → $YZ

5. Run unit tests: npm test in the variable package

[erunion]

ill publish this tomorrow morning