fix: align --locked metadata serializer with lock-write side

[baszalmstra]

Description

Follow-up to #6052. When [tool.uv.sources] redirects an optional-dep to a named index, uv lowering tags the registry source with index = Some(...) and Requirement::Display emits (index: <url>). The lock-write path (pixi_uv_conversions::to_requirements) strips that suffix; the --locked satisfiability path was reparsing req.to_string() with pep508_rs, which rejects it. Result: --locked errored on a lockfile pixi had just written.

Route the satisfiability path through the same to_requirements helper so both sides serialize identically.

Fixes #6049 (follow-up reported by @diegoferigo-rai on rai-opensource/exploy).

How Has This Been Tested?

• New unit test to_requirements_strips_index_suffix_from_registry_source constructs a Requirement with index = Some(...), asserts uv's Display includes (index:, and pins to_requirements output to plain isaaclab.
• cargo test -p pixi_core --lib lock_file::satisfiability; 150 passed, no regressions on existing test_good_satisfiability / test_failing_satisfiability cases.

AI Disclosure

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  • I have tested any AI-generated content in my PR.
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Tools: Claude (claude-opus-4-7, via Claude Code on web)

Checklist:

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