feat: implement uv-style dependency sorting and multiline formatting

[kajal-jotwani]

Description

When running pixi add --pypi with multiple packages, dependencies were previously written as inline arrays with inconsistent ordering, which could create unnecessary diffs when removing and re-adding packages.

This PR updates PyPI dependency formatting to follow uv-style behavior:

• arrays are always written in multiline format (one dependency per line)
• if the existing list is alphabetically sorted, new dependencies are inserted in order
• if not, the existing order is preserved and new dependencies are appended

The multiline formatting logic is adapted from uv’s reformat_array_multiline, which preserves inline and trailing comments.

Fixes #5651

How Has This Been Tested?

Added three unit tests covering sorting behavior, multiline formatting, and the remove-and-readd zero-diff case. Snapshot tests were updated accordingly.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
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Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[baszalmstra]

Can you add unit tests specifically for reformat_array_multiline? Especially with some edge cases?

[kajal-jotwani]

Can you add unit tests specifically for reformat_array_multiline? Especially with some edge cases?

Thanks for the review! I’ve added unit tests for reformat_array_multiline, covering edge cases like empty arrays, idempotency, and preservation of different comment types (own-line, end-of-line, and trailing) and inline comment spacing.