Fix HtmlToDjot block structure edge cases

[dereuromark]

Summary

• fix

   conversion so nested descendant  elements do not hijack the block and language names like c++ are preserved

• preserve separate lines in line-block containers when the HTML uses sibling paragraphs
• preserve paragraph boundaries in blockquotes without leaking whitespace-only text nodes into the Djot output
• add converter regressions for the new block-structure cases plus the missing HtmlToDjot edge cases now covered on master

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

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.91%. Comparing base (2ee23fb) to head (31b9d81).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Converter/HtmlToDjot.php	95.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #164      +/-   ##
============================================
+ Coverage     93.83%   93.91%   +0.08%     
- Complexity     3067     3075       +8     
============================================
  Files            93       93              
  Lines          7881     7891      +10     
============================================
+ Hits           7395     7411      +16     
+ Misses          486      480       -6     

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