fix(pixi-build-python): clarify missing pyproject error

[lawrence3699]

Closes #5871

When pixi-build-python is used without a pyproject.toml, metadata loading currently fails with a raw file-open error while reading package metadata.

This changes the metadata provider to surface a dedicated error that states that pixi-build-python requires a pyproject.toml and points at the missing path.

Validation:

• cargo fmt --all --check
• cargo test -p pixi-build-python metadata::tests:: -- --nocapture

[Copilot]

Pull request overview

This PR improves UX for pixi-build-python by surfacing a dedicated, actionable error when a project is missing pyproject.toml, instead of bubbling up a low-level file I/O error during metadata loading.

Changes:

• Added a MissingPyProjectToml MetadataError variant with a clearer message and diagnostic help text.
• Updated manifest loading to map ErrorKind::NotFound to the new dedicated error.
• Added a regression test asserting both the error variant and message content when pyproject.toml is absent.

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[baszalmstra]

Thanks this is a great ux improvement!

[baszalmstra]

Perhaps one improvement would be if we could surface this error earlier, I think we could surface this in the first few lines of recipe generation. Because now the error still includes that it could read a certain metadata like the homepage. But the backend cannot work without a pyproject.