feat(py-rattler): expose write_repodata for wheel metadata

[officialasishkumar]

Description

Expose rattler.index.write_repodata(...) in py-rattler so Python callers can write repodata.json files with v3.whl entries directly from WhlPackageRecord metadata.

Issue #2211 asked for three Python-side pieces:

• WhlPackageRecord(...)
• PackageRecord.extra_depends
• write_repodata(channel_dir, whl_records, write_zst, write_shards)

The first two are already available on main; this PR adds the remaining async write_repodata binding, exports it from rattler.index, and covers it with a focused unit test.

Fixes #2211

How Has This Been Tested?

From the repository root:

• pixi run cargo-fmt
• pixi run cargo-clippy

From py-rattler/:

• pixi run fmt-python-check
• pixi run build
• pixi run -e test -x pytest tests/unit/test_index.py -k write_repodata -q

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Codex

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.