Fix compatibility report parser for 4-backtick fences

[dereuromark]

Summary

Fixes the compatibility report's test file parser to correctly parse official djot test files that use 4-backtick fences.

The Problem

The compatibility report was showing 2 failing tests (99% compatibility), but investigation revealed the actual djot parser handles these cases correctly. The issue was in how tests/compatibility-report.php parses the official test files.

Many djot tests use 4-backtick fences (``````) to wrap test cases that contain 3-backtick code blocks (```). The parser's regex only matched 3-backtick fences, causing it to incorrectly parse nested content.

The Fix

Updated the regex to match both fence lengths:

// Before: only matched 3 backticks
preg_match_all('/```\n(.*?)\n```/s', ...)

// After: matches both, longer first for proper nesting
preg_match_all('/````\n(.*?)\n````|```\n(.*?)\n```/s', ...)

Result

• Before: 236/238 tests (99% compatibility) - false failures
• After: 245/245 tests (100% compatibility)

The increase in test count (238 → 245) is because tests wrapped in 4-backtick fences are now correctly parsed, and some test files had additional tests that were previously missed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.68%. Comparing base (5085c8e) to head (8806673).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #52      +/-   ##
============================================
+ Coverage     93.64%   93.68%   +0.04%     
- Complexity     1916     1928      +12     
============================================
  Files            74       74              
  Lines          5143     5177      +34     
============================================
+ Hits           4816     4850      +34     
  Misses          327      327              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.