Fix uv PEX builder to use pex3 lock export

[seungwoo-ji-03]

When the uv PEX builder extracts pinned requirements from a PEX-native lockfile, it previously parsed the internal JSON format directly. This broke VCS and direct URL requirements (formatted as name==version instead of name @ url), and the PEX maintainer has warned that this internal format is unsupported and may change without notice.

This PR replaces JSON parsing with pex3 lock export --format pep-751 via PexCliProcess. The exported pylock.toml is passed directly to uv pip install -r, handling VCS refs, direct URLs, extras, and platform filtering out of the box.

Follow-up to #23197 (reported by @benjyw, approach suggested by @jsirois).
Supersedes #23218.

[seungwoo-ji-03]

Chose pep-751 over pip-no-hashes for two reasons:

1. Direct URL sdist preservation: pep-751 preserves direct URL sdist references via archive sections (built from the lockfile's input requirements), while pip-no-hashes drops them to name==version in export output.

2. VCS commit pinning: pep-751 requires commit_id on VCS artifacts, which is populated when using --pip-version 24.2 (Pants default). Locks created with older pip/Pex may fail export when VCS artifacts are missing commit_id (e.g. 20.3.4-patched); in Pants, the code falls back to transitive uv resolution in that case.

(Based on reading lock.py and pep_751.py)

[jsirois]

@seungwoo-ji-03 this looks good to me, but a few notes - the 2nd - the existence of pex3 lock export-subset, may prove useful at some point.

[benjyw]

The fix is fine, but the new tests aren't testing much - everything interesting is mocked out. Better to add some URL requirements to some existing tests, and/or create new ones that work the same way (via rule_runner.request() and/or the create_pex_and_* helpers).

[benjyw]

There is no need to call out a bugfix in a feature that hasn't been released yet (there has been no 2.32.0 stable yet). From the user perspective this bug was never in the wild. So you can revert this.