fix: reuse existing tool.pixi.feature pypi-dependencies table

[Premkumar-2004]

Description

pixi add --pypi -f <feature> was always creating [dependency-groups] entries even when [tool.pixi.feature.<feature>.pypi-dependencies] already existed. Fixed by checking for existing table first.

Fixes #5492

How Has This Been Tested?

Added tests for both cases - reusing existing table and fallback to dependency-groups.

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: Claude

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.

[ruben-arts]

This works, thank you