ci(release): publish PEP 740 attestations to PyPI for hf-xet (#848)

## Summary

Switches the `release-pypi` job from `PyO3/maturin-action`'s upload
command to `pypa/gh-action-pypi-publish` with `attestations: true`, so
that every `hf-xet` artifact on PyPI ships with **PEP 740 / Sigstore
provenance**.

Today, `hf-xet 1.5.0` on PyPI shows `attestations: None` and
`https://pypi.org/integrity/hf-xet/1.5.0/<wheel>/provenance` returns
404. The existing `actions/attest-build-provenance` step does the work,
but its output never reaches PyPI: `gh attestation verify` works for
wheels people downloaded from GitHub releases, but `pip download
--verify-attestations` (and the equivalent uv / SLSA verifier flows)
have nothing to chain to.

## What changes

In `.github/workflows/release.yml`, `release-pypi` job:

- Add a step that collects every `wheels-*/<file>` artifact into a
single `dist/` directory. Debug-symbol artifacts (`dbg-*`) are
explicitly excluded from this directory and therefore from the upload.
- Point `actions/attest-build-provenance` at `dist/*` so the GitHub
attestation covers exactly the same bytes that get uploaded.
- Replace `PyO3/maturin-action@... command: upload` with
`pypa/gh-action-pypi-publish@cef221092ed1bacb1cc03d23a2d87d1d172e277b`
(v1.14.0, SHA-pinned) and `attestations: true`, `skip-existing: true`,
`print-hash: true`.

## Why two attestation steps?

They are complementary, not redundant:

| Step | Verifier | Where it lives |
|---|---|---|
| `actions/attest-build-provenance` | `gh attestation verify`,
sigstore-go | github.com attestations endpoint |
| `pypa/gh-action-pypi-publish` with `attestations: true` | `pip
download --verify-attestations`, uv, slsa-verifier | PyPI
`/integrity/.../provenance` |

The first one verifies wheels distributed anywhere; the second one is
what PyPI clients actually look at.

## Verification after merge

On the first release that runs this workflow:

```bash
$ pip download --no-deps hf-xet==<new-version>
$ python -c "from pypi_attestations import Distribution; print(Distribution.from_file('hf_xet-*.whl').identity)"
# should print the workflow / environment / repo identity
```

And on PyPI's web UI: each wheel should show a "Provenance" link instead
of "no attestations".

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes the release publishing path and artifact selection for PyPI
uploads; mistakes could block releases or upload the wrong artifacts
despite being confined to CI.
> 
> **Overview**
> Switches the `release-pypi` job to **collect all wheel/sdist artifacts
into a single `dist/` directory** (excluding `dbg-*`) and generate the
GitHub build provenance attestation over exactly those files.
> 
> Replaces the PyPI upload step from `PyO3/maturin-action` to
`pypa/gh-action-pypi-publish` using **Trusted Publishing + PEP 740
attestations** (`attestations: true`, `skip-existing: true`,
`print-hash: true`), and clarifies required workflow permissions.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
4f0c7f8db9. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

.github/workflows/release.yml

@@ -379,24 +379,32 @@ jobs:
     if: ${{ github.event_name == 'workflow_dispatch' }}
     needs: [github-release]
     permissions:
-      # Use to sign the release artifacts
+      # Used by both attest-build-provenance and the PyPI Trusted Publisher (OIDC).
       id-token: write
-      # Used to upload release artifacts
+      # Used to upload release artifacts and to generate artifact attestations.
       contents: write
-      # Used to generate artifact attestation
       attestations: write
     environment: release
     steps:
       - uses: actions/download-artifact@37930b1c2abaa49bbe596cd826c3c89aef350131  # v7
-      - name: Generate artifact attestation
+      - name: Collect wheels and sdist into dist/
+        run: |
+          set -euo pipefail
+          mkdir -p dist
+          # Each `wheels-*` artifact directory holds one or more wheels/sdists.
+          # Debug-symbol artifacts (`dbg-*`) must NOT be uploaded to PyPI.
+          cp wheels-*/* dist/
+          ls -l dist
+      - name: Generate GitHub build provenance attestation
         uses: actions/attest-build-provenance@977bb373ede98d70efdf65b84cb5f73e068dcc2a  # v3
         with:
-          subject-path: 'wheels-*/*'
-      - name: Publish to PyPI
-        uses: PyO3/maturin-action@04ac600d27cdf7a9a280dadf7147097c42b757ad  # v1
+          subject-path: 'dist/*'
+      - name: Publish to PyPI (Trusted Publishing + PEP 740 attestations)
+        uses: pypa/gh-action-pypi-publish@cef221092ed1bacb1cc03d23a2d87d1d172e277b  # v1.14.0
         with:
-          command: upload
-          args: --non-interactive --skip-existing wheels-*/*
+          attestations: true
+          skip-existing: true
+          print-hash: true
 
   github-release:
     name: Create GitHub release