feat(packagesettings): error when source and legacy settings are both set

[LalatenduMohanty]

Summary

• Add Pydantic model validators on PackageSettings and VariantInfo that reject configs combining the new source field with legacy settings
• Package-level source conflicts with download_source and resolver_dist; variant-level source conflicts with wheel_server_url and pre_built
• Uses model_fields_set to correctly detect explicitly provided legacy fields even when they have defaults

Test plan

• Parametrized tests for invalid package-level combos (source + download_source, source + resolver_dist, both, empty download_source)
• Parametrized tests for invalid variant-level combos (source + wheel_server_url, source + pre_built)
• Valid legacy-only and source-only configs still load
• Error message includes conflicting field names
• mypy and lint pass

Closes: #1207

🤖 Generated with Claude Code

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[rd4398]

This looks good! I will let @smoparth take a look as well

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