Add default-conda-pypi-map to Config.toml.

[FHTMitchell]

This sets conda-pypi-map on pixi-init from the default values in config (unless overridden with the CLI flag).

Description

Currently, you can add a [conda-pypi-map] section to a pixi.toml manifest.

This works, but means it needs to be updated per repo in an organisation. This PR adds a config section to the ~/.pixi/config.toml, [default-conda-pypi-map] that writes then conda-pypi-map to the pixi.toml on pixi init, much like the (default-)channels section.

Fixes #6856

Open Questions For Reviewer

1. This moves CondaPypiMap and associated types from pixi_manifest to pixi_config. pixi_config is already a transitive dependency of pixi_manifest so this doesn't change anything in practise, but should we move this out into another leaf crate instead?
2. Should we add the wildcard feature I mentioned in the issue? Or in another PR?

How Has This Been Tested?

I have run

cargo test -p pixi_config -p pixi_manifest -p pixi_api

which pass. I actually can't build the full repo locally due to upstream errors.

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.

Claude Code was used but I iterated with it manually and have reviewed the entire output to my standards. The issue and this PR description was entirely written by me.

Tools: Claude Code

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.