feat(resolver): add download method to all resolver classes

[tiran]

Pull Request Description

What

Add _download() to AbstractResolver that dispatches to the
appropriate download helper based on a mandatory download_kind
argument. Each concrete resolver implements a public download()
that delegates with its specific DownloadKind.

• PyPIDownloadResolver accepts download_kind as a mandatory
  Pydantic field (restricted to sdist | tarball)
• NotAvailableResolver and AbstractHookResolver override
  download() to raise explicitly
• Replace the old download_kind ClassVar with a download_kinds
  frozenset ClassVar describing which kinds each resolver can return
• Replace resolves_prebuilt_wheel property with a ClassVar
• Validate download_kind against download_kinds in _download()

Why

Include the download logic in the resolver.

• PR follows CONTRIBUTING.md guidelines

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📥 Commits

Reviewing files that changed from the base of the PR and between c378fa9 and ed032d1.

📒 Files selected for processing (3)

• docs/reference/config-reference.rst
• src/fromager/packagesettings/_resolver.py
• tests/test_packagesettings_resolver.py

🚧 Files skipped from review as they are similar to previous changes (2)

• tests/test_packagesettings_resolver.py
• src/fromager/packagesettings/_resolver.py

---

📝 Walkthrough

Walkthrough

This PR replaces resolver download_kind metadata with download_kinds sets in _resolver.py, adds a DownloadKindSet alias, and moves download handling into a centralized _download() dispatcher. Concrete resolvers now implement download() and advertise supported kinds explicitly. Tests were updated to use shared Candidate fixtures, mock downloader helpers, assert (path, kind) results, and verify the new error paths. The reference docs now list download_kinds and resolves_prebuilt_wheel.

Estimated code review effort: 3 (Moderate) | ~25 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: adding a download method across resolver classes.
Description check	✅ Passed	The description accurately describes the resolver download refactor and is clearly related to the changeset.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/fromager/packagesettings/_resolver.py`:
- Around line 300-301: The download_kind field in the resolver settings is now
required but existing pypi-download configs may omit it, so either give it a
backward-compatible default in the relevant resolver settings class or treat
this as an intentional config break and document it with a versionchanged note
in the class docstring. Update the field definition in the package settings
resolver so loading configs that don’t specify download_kind continues to work,
or clearly signal the breaking change if compatibility is not preserved.

In `@tests/test_packagesettings_resolver.py`:
- Around line 302-315: The resolver test coverage is missing the `download_kind:
sdist` execution path, so `download()` is never verified to dispatch through
`download_sdist` instead of the tarball helper. Add a new test alongside
`test_download` that configures the resolver with `download_kind: sdist`,
patches `fromager.downloads.download_sdist`, and asserts the returned path/kind
plus the expected call arguments. Use the existing `download()` method and
`_parse()` setup to keep the new case aligned with the current resolver tests.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 881e8a75-c367-4165-807e-7dc692fd2e20

📥 Commits

Reviewing files that changed from the base of the PR and between dbcd337 and c378fa9.

📒 Files selected for processing (2)

• src/fromager/packagesettings/_resolver.py
• tests/test_packagesettings_resolver.py

[rd4398]

Overall looks good. I added couple of comments / suggestions

[rd4398]

This looks good! Thank you

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