ci: constrain internal builds to exact local wheels

[rwgk]

Description

Fixes #2468.

xref: #2464, #2470

Same-checkout CI builds must use the exact CUDA Python wheel artifacts selected by the workflow. A compatible package from PyPI must not satisfy an internal build dependency when CI intends to exercise a local artifact.

This PR generates PEP 508 direct-reference constraints for the selected cuda-pathfinder and cuda-bindings wheels, for example:

cuda-pathfinder @ file:///.../cuda_pathfinder-...whl
cuda-bindings @ file:///.../cuda_bindings-...whl

The workflows apply each constraint file through both PIP_BUILD_CONSTRAINT and PIP_CONSTRAINT. This constrains dependencies installed into PEP 517 isolated build environments as well as other pip dependency resolution in the build step, while leaving package indexes available for unrelated third-party dependencies.

PRs #2509 and #2520 report the versions and import locations of build dependencies. Those diagnostics make the selected artifacts visible in CI logs; the constraints in this PR enforce their selection.

Dependency constraints

The protected build edges are:

Consumer build	Required wheel artifacts
cuda.bindings	exactly one workflow-built cuda-pathfinder wheel
current-major cuda.core	exactly one cuda-pathfinder wheel and one cuda-bindings wheel matching the current CUDA major
previous-major cuda.core	exactly one cuda-pathfinder wheel and one downloaded cuda-bindings wheel matching the previous CUDA major

Each constraint-generation step verifies that exactly one matching artifact exists before starting the consumer build. The previous-major bindings artifact is kept in a separate directory so current- and previous-major wheels cannot become an ambiguous input set.

The direct references use paths appropriate to each execution environment:

• Linux cibuildwheel builds use the container-visible /host mount.
• Windows builds convert paths with cygpath for Git Bash and native pip subprocesses.
• Native Linux workflow steps use absolute file URLs.

Affected environments install pip 25.3 or newer for build-constraint support.

Workflow coverage

The constraints are applied consistently across:

• production Linux and Windows wheel builds, including the previous-CUDA-major cuda.core build;
• Linux and Windows coverage builds; and
• Linux and Windows wheel-from-sdist tests.

The policy is contained entirely in CI configuration. Package build requirements and ordinary builds without these constraint variables are unchanged.

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[rwgk]

/ok to test

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[github-actions]

Doc Preview CI
🚀 View preview at https://nvidia.github.io/cuda-python/pr-preview/pr-2510/
https://nvidia.github.io/cuda-python/pr-preview/pr-2510/cuda-core/
https://nvidia.github.io/cuda-python/pr-preview/pr-2510/cuda-bindings/
https://nvidia.github.io/cuda-python/pr-preview/pr-2510/cuda-pathfinder/
Preview will be ready when the GitHub Pages deployment is complete.

[rwgk]

@kkraus14 This PR now keeps dependency selection entirely in CI, uses exact constraints for both pathfinder and bindings across every in-scope workflow, and no longer changes either build hook. I plan to merge after clean CI unless you see a remaining blocker.

(At this moment the CI is very unreliable b/o infrastructure issues; I'll keep retrying; we had a clean run before I merged 2520, therefore I believe it should work again.)

[kkraus14]

Reposting comment I made offline:

There is so much unnecessary added complexity here. We shouldn't need a whole python helper for constraint file generation. We already know the exactly wheel we built earlier for dependencies so we can easily just pass that information through in the GitHub Action workflows.

[rwgk]

@kkraus14, I removed the Python helper and moved the constraint generation directly into the workflows, following your suggestion. CI is running, and I've updated the PR description to reflect the new implementation. Could you please take another look?