fix: abi3 run export handling for pixi-build-python

[pavelzw]

Description

Fix ABI3 dependency handling in pixi-build-python.

When abi3 = true, pixi was still producing a python_abi X.Y.* *_cpXY run dependency via the weak run export from host: python, even though the recipe was marked build.python.version_independent: true.

Root cause

pixi-build-python was mixing two different models for ABI3 packages:

• it set build.python.version_independent = true
• but it also treated ABI3 like a normal CPython extension build by adding python_abi logic / allowing host: python run exports to propagate

That caused regular CPython ABI pins to still appear in the resolved run dependencies.

Changes

• stop treating abi3 = true as a python_abi host pin
• for ABI3 builds, add ignore_run_exports.from_package = ["python"] so host: python does not inject the normal python_abi run export
• extend the stage0/intermediate recipe model to carry and serialize ignore_run_exports
• update pixi-build-python docs to describe the correct ABI3 behavior
• update tests to cover the new generated recipe behavior

Result

ABI3 builds now:

• remain version_independent
• no longer inherit python's CPython ABI pin
• rely on the explicit python-abi3 host dependency for ABI3-compatible run exports

Fixes conda/rattler#2197 (comment)

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: codex

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]

Let me check this on Tuesday. I think we still need the lower/upperbound check.

[pavelzw]

i also still need to look at the code here, this was entirely done by codex apart from my prompt