Fix HtmlToDjot MathML extraction edge cases

[dereuromark]

Summary

• ignore non-TeX MathML annotation payloads when falling back to plain extracted math text
• use a safe backtick fence for extracted math content so TeX containing backticks remains valid Djot
• add converter regressions for both MathML edge cases

Verification

• vendor/bin/phpunit tests/TestCase/Converter/HtmlToDjotTest.php
• vendor/bin/phpunit tests/TestCase/Converter/HtmlToDjotTest.php tests/TestCase/RoundTripTest.php tests/TestCase/DjotConverterTest.php
• vendor/bin/phpstan analyse src --no-progress
• vendor/bin/phpunit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.96%. Comparing base (53588fb) to head (73316bf).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #166      +/-   ##
============================================
+ Coverage     93.94%   93.96%   +0.01%     
- Complexity     3106     3113       +7     
============================================
  Files            93       93              
  Lines          7950     7964      +14     
============================================
+ Hits           7469     7483      +14     
  Misses          481      481              

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