Fix MarkdownRenderer blockquote child spacing

[deepakganesh78]

Fixes #368

Reproduction

On current main, reformatting a block quote with adjacent child blocks inserts an extra quoted blank line:

import mistune
from mistune.renderers.markdown import MarkdownRenderer

md = mistune.create_markdown(renderer=MarkdownRenderer())
print(repr(md("> #### Lorem\n> Ipsum")))

Before this change the result was '> #### Lorem\n> \n> Ipsum\n'.

Root cause

MarkdownRenderer.block_quote() rendered its child tokens as top-level Markdown before adding quote markers. Top-level block renderers intentionally end blocks with blank separators, so adjacent child blocks inside the same quote gained an extra > line even when the parsed quote AST did not contain a blank_line token.

Fix

Render block quote children token-by-token and collapse only the structural separator between adjacent non-blank child tokens before applying the quote prefix. Explicit blank_line tokens are preserved, so quoted paragraph breaks continue to render with a quoted blank line.

Compatibility

This only changes MarkdownRenderer output for adjacent child blocks inside block quotes. HTML rendering and explicit blank lines within block quotes are unchanged.

Validation

• python -m pytest tests\test_renderers.py::TestMarkdownRendererRoundTrip::test_block_quote_adjacent_child_blocks -q -> 1 passed
• With the renderer fix reverted, the new regression test fails with '> #### Lorem\n> \n> Ipsum\n' != '> #### Lorem\n> Ipsum\n'
• python -m pytest -> 765 passed
• python -m ruff check . -> All checks passed
• python -m mypy -> Success: no issues found in 38 source files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.21%. Comparing base (75cab78) to head (644448d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #472      +/-   ##
==========================================
+ Coverage   91.18%   91.21%   +0.02%     
==========================================
  Files          36       36              
  Lines        3631     3642      +11     
  Branches      677      679       +2     
==========================================
+ Hits         3311     3322      +11     
  Misses        193      193              
  Partials      127      127              

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