Implement --with-credentials flag in UV export_requirements

[Copilot]

The with_credentials parameter was accepted but silently ignored in UV's export_requirements method. This breaks PackageExtractor when using UV with private packages, since it calls export_requirements(..., with_credentials=True) expecting credentials to be embedded in the output.

Changes

• versifier/uv.py: Pass --with-credentials flag to uv export when with_credentials=True
• tests/test_uv.py: Assert flag presence/absence in both credential scenarios

# Before: credentials silently dropped
uv.export_requirements(with_credentials=True)  # Flag ignored

# After: credentials properly embedded
uv.export_requirements(with_credentials=True)  # Adds --with-credentials to uv export

Matches existing Poetry behavior where --with-credentials embeds authentication data in requirements.txt for private indices.

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