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.

[tdejager]

Hey @aryanyk! Thanks for the PR can you fill out the template correctly? :)

[aryanyk]

Hi @tdejager

Thanks for the reminder!
I’ve updated the PR description to fully match the template and added testing details.

Description
Fixes #4587

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

What’s changed

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

How Has This Been Tested?
AI Disclosure

• This PR contains AI-generated content.
• I have tested any AI-generated content in my PR.
• I take responsibility for any AI-generated content in my PR.
• Tools: {e.g., Claude Sonnet 4.5 ,ChatGPT 5.2}

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.

[aryanyk]

Thanks for the review!

Just to clarify: this PR (#5151) is the cleaned-up, non-draft implementation intended for merge.
The earlier draft PR was exploratory and can be ignored.

I’ve updated the PR description to fully match the template.
Happy to investigate or rerun CI if you’d like me to dig into the pixi-build failures.