Use safe blockquote prefix on legacy Windows

[lwhelton]

Type of changes

• [X ] Bug fix
• New feature
• Documentation / docstrings
• [X ] Tests
• Other

AI?

• AI was used to generate this PR

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Checklist

• [X ] I've run the latest black with default args on new code.
• [X ] I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate (see note about typos above).
• [X ] I've added tests for new code.
• [ X] I accept that @willmcgugan may be pedantic in the code review.

Description

Please describe your changes here.
Fixes Markdown blockquote rendering on legacy Windows / restricted encodings by falling back from ▌ to | when the output environment is not safe for the Unicode block character. Normal environments still use the original blockquote marker. A regression test was added to verify the safe fallback behavior.

Important: Link to an issue or discussion regarding these changes.