fix(dependencies): use the sdist requirements hook

[mikedep333]

Pull Request Description

What

Use get_requires_for_build_sdist() when discovering sdist dependencies,
and correct the associated docstring.

Add regression tests using real backend subprocess calls covering distinct
wheel and sdist requirements, an absent optional sdist hook, and a failing
wheel hook. All three cases fail before the fix and pass afterward.

Existing cached sdist requirements are not invalidated.

Why

Sdist dependency discovery currently calls the wheel requirements hook.
This can omit sdist-only dependencies, record wheel dependencies as sdist
requirements, and unnecessarily execute wheel-specific backend logic.

Closes #1351.

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Reviewing files that changed from the base of the PR and between 27db2d6 and c729f36.

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• src/fromager/dependencies.py
• tests/test_dependencies.py

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📝 Walkthrough

Walkthrough

default_get_build_sdist_dependencies now calls get_requires_for_build_sdist instead of the wheel requirements hook. Tests cover distinct hook results, a failing wheel hook, and an absent sdist hook.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to c729f

The change aligns source-build dependency discovery with the sdist hook while preserving existing cache behavior. No actionable merge-blocking risk is identified; merge after normal checks pass.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: using the sdist requirements hook for dependency discovery.
Description check	✅ Passed	The description directly explains the hook change, affected behavior, regression tests, rationale, and linked issue.
Linked Issues check	✅ Passed	Issue #1351 requires default_get_build_sdist_dependencies() to use get_requires_for_build_sdist(), return its requirements, return an empty list when the optional hook is absent, and avoid the whe…
Out of Scope Changes check	✅ Passed	The changes stay within Issue #1351. The production change fixes the hook selection. The added tests verify the required behavior through pyproject_hooks subprocess calls. No unrelated source or tes…

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[mikedep333]

@dhellmann I talked to tiran at the meeting, and he suggested asking you if there was an intentional reason why default_get_build_sdist_dependencies originally called get_requires_for_build_wheel