ci: prep for 3.15t — libhdf5-dev on pre-release Python + scikit-learn>=1.9.1

[priya-sundaram-dev]

Splits the non-3.15t-gated changes out of #15105 so they can land before GA, as requested by @cclauss.

Describe your change:

CI/infra only (no algorithm changes):

• .github/workflows/build.yml / .github/workflows/sphinx.yml: install libhdf5-dev only when the interpreter is a pre-release (releaselevel != final) — keras needs hdf5 there. Guarded no-op on final Python, so nothing changes for current CI.
• pyproject.toml: pin scikit-learn>=1.9.1, the first release shipping cp315t free-threaded wheels, so uv resolves a wheel rather than building from source.

Deliberately not included here: the .python-version bump 3.14t → 3.15t, which stays in #15105 as a draft until Python 3.15 GA.

Ref #15081, #15105.

• Other (CI / build configuration only — no algorithm change)

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes CI/build configuration files; it does not add or modify an algorithm.

[algorithms-keeper]

Closing this pull request as invalid

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