Summary

Add support for storing PyPI prerelease mode per environment in lock files. This enables tools like pixi to persist their prerelease configuration.

Related to: prefix-dev/pixi#4984

Changes

• Add PypiPrereleaseMode enum to SolveOptions with 5 variants:
  • Disallow - Disallow all pre-release versions
  • Allow - Allow all pre-release versions
  • IfNecessary - Allow pre-release versions if necessary to satisfy requirements
  • Explicit - Allow pre-release versions for packages explicitly requested
  • IfNecessaryOrExplicit - Default mode (allow if necessary or explicitly requested)
• Add optional pypi_prerelease_mode field to SolveOptions struct
• Add builder methods: set_pypi_prerelease_mode() and with_pypi_prerelease_mode()
• Add Environment::pypi_prerelease_mode() getter method
• Add tests for serialization roundtrip of all prerelease modes

Lock File Format

The prerelease mode is stored at the environment level within the options section:

version: 6
environments:
  default:
    channels: []
    options:
      pypi-prerelease-mode: allow
    packages:
      linux-64:
      - conda: https://...

Backwards Compatibility

The implementation is fully backwards compatible:

• The pypi_prerelease_mode field is optional (Option<PypiPrereleaseMode>)
• Uses skip_serializing_if = "Option::is_none" so old lock files without this field will parse correctly
• Uses default attribute so missing field deserializes to None
• Existing roundtrip tests all pass, confirming no breaking changes