ci: trim Python PR matrix and add sccache

[pratyush618]

Summary

• The Python job was CI's long pole: 9 matrix cells, each doing a full --release maturin build (all heavy features) plus a ~10-minute serial pytest, with near-zero Cargo-cache reuse (the lint job writes dev-profile artifacts; these builds are release-profile).
• PRs now run a trimmed 3-cell matrix — ubuntu × {3.10, 3.14} plus one windows-2025 × 3.14 smoke covering the vendored-OpenSSL path. The full 9-cell version/OS matrix still runs on every push to master and on workflow_dispatch, so coverage is delayed to merge, not lost.
• Adds sccache (mozilla-actions/sccache-action, GHA cache backend) to the matrix job so the release builds share compiled rustc artifacts across cells and runs.

Changes

• .github/workflows/ci-python.yml only: conditional strategy.matrix on github.event_name, SCCACHE_GHA_ENABLED/RUSTC_WRAPPER job env, sccache setup step before the maturin build.

Test plan

• This PR's own run shows the 3-cell matrix
• Post-merge master push shows the full 9-cell matrix
• Second PR run after cache warm-up shows reduced maturin build time (sccache hits)
• YAML validates; actionlint runs in CI

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: d408fc2b-b729-487c-8b9e-1725889e7620

📥 Commits

Reviewing files that changed from the base of the PR and between 943f4a3 and 6e05e29.

📒 Files selected for processing (1)

• .github/workflows/ci-python.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/trim-python-pr-matrix

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