fix: don't misclassify missing python as pypi-only during upgrade

[baszalmstra]

Description

pixi upgrade on a workspace with a local editable PyPI dep (e.g. pkg = { path = ".", editable = true }) failed on the second run with No Python interpreter found in the dependencies.

Root cause: update_dependencies unlocks python from the conda lock before re-solving. Satisfiability then calls read_local_package_metadata for the path source, which looks up python in locked_pixi_records first and on miss returned FailedToReadLocalMetadata. FailedToReadLocalMetadata marked the satisfiability as "is_pypi_only" variant. Because the conda dep walk runs after the pypi metadata read inside the try_join! in verify_package_platform_satisfiability, that pypi-only error short-circuited the join, the env was marked only outdated_pypi, conda was never re-solved, and the subsequent pypi resolve aborted on the missing python record.

Fix: return the existing non-pypi-only PlatformUnsat::MissingPythonInterpreter in that branch, and stop re-wrapping read_local_package_metadata errors at the call site (the wrap was discarding variant info). The conda walk now correctly schedules a re-solve, python lands back in the records, and the pypi resolve proceeds normally.

Fixes #6093

How Has This Been Tested?

cargo check -p pixi_core passes.

Manual repro from the issue:

[workspace]
channels = ["conda-forge"]
name = "hello_world"
platforms = ["linux-64"]

[dependencies]
python = ">=3.14.4,<3.15"

[pypi-dependencies]
hello_world = { path = ".", editable = true }

Run pixi upgrade twice, before this change, the second run errors, after it succeeds.

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 (Opus 4.7)

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.

[ruben-arts]

Thank you