feat: support lists inside blockquotes

[forest6511]

Summary

• Adds structured QuoteContent model with QuoteParagraph and QuoteList block types, following the same polymorphic pattern as FencedDivContent
• Refactors Helpers.GetQuoteRuns() → GetQuoteContent() to extract both paragraphs and lists from blockquotes
• Each list item renders as a separate paragraph with full quote styling (border, background, indentation)

Closes #70

Changes

File	Change
QuoteContent.cs	New model: QuoteContent, QuoteContentBlock, QuoteParagraph, QuoteList
Helpers.cs	GetQuoteRuns → GetQuoteContent with ListBlock switch case
IDocumentBuilder.cs	AddQuote(IReadOnlyList<InlineRun>, ...) → AddQuote(QuoteContent, ...)
OpenXmlDocumentBuilder.cs	New AddQuoteParagraph + AddQuoteList private methods
Program.cs	Updated dispatch to use GetQuoteContent

Test plan

• 309 tests passing (302 existing + 7 new)
• New parser tests: list inside quote, paragraph+list, ordered list, plain text (regression)
• New builder tests: unordered list, ordered list, mixed content, empty content
• All existing quote tests updated and passing (null checks, padding, borders, bold/code runs)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.88%. Comparing base (ba18498) to head (52d821d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
+ Coverage   88.69%   88.88%   +0.19%     
==========================================
  Files          27       28       +1     
  Lines        1424     1449      +25     
  Branches      179      183       +4     
==========================================
+ Hits         1263     1288      +25     
  Misses        122      122              
  Partials       39       39              

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