feat(add): support per-dependency PyPI index via --index

[aryanyk]

Context

This PR implements support for specifying a custom PyPI index when adding a PyPI dependency via the pixi add command, as requested in #4587.

Currently, users must manually edit the manifest to configure a non-default PyPI index. This change brings Pixi in line with tools like uv add by allowing the index to be specified directly at add-time.

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What’s changed

• Added a new --index <URL> parameter to pixi add --pypi
• Wired the provided index URL into the generated PixiPypiSpec for the added dependency
• Ensured the change is per-dependency, not workspace-wide
• Added an integration test validating that the index is persisted correctly in the manifest

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Example

pixi add --pypi requests --index https://example.com/simple


### Checklist:

-  I have performed a self-review of my own code
-  I have commented my code, particularly in hard-to-understand areas
-  I have added sufficient tests to cover my changes.

[baszalmstra]

Can we close this in favor of #5151 ?