Consolidate Python discovery strategies

[zanieb]

Summary

Consolidate the sequential and parallel Python discovery paths introduced in #18684.

Both modes now share a single PythonRequest dispatcher and executable-to-installation pipeline. A private QueryStrategy selects either lazy sequential querying or eager parallel querying while preserving the existing filtering, ordering, and error-handling behavior.

No prefetching is introduced here. Sequential discovery still defers reading and traversing PATH, advances candidate executables one at a time, and stops querying after finding an acceptable installation. The pre-existing eager behavior for explicit file, directory, and executable-name requests is unchanged.

Test Plan

• Added parity coverage across every PythonRequest variant, using independent caches and out-of-order query completion to verify ordered results and non-critical error handling.
• Added regression coverage proving sequential strategy construction pulls no candidates, one iteration pulls exactly one candidate, later PATH directories are not inspected early, and later interpreters are not queried after a match.
• Mutation-checked the laziness tests against candidate collection, eager interpreter querying, and eager PATH directory filtering.
• cargo test -p uv-python
• cargo test -p uv --test python python_list --no-fail-fast
• cargo test -p uv --test python python_find --no-fail-fast
• cargo +stable clippy -p uv-python --tests --no-deps --locked -- -D warnings
• cargo +stable fmt --all -- --check

[astral-sh-bot]

uv test inventory changes

This PR changes the tests when compared with the latest main baseline.

• Added tests: 4
• Removed tests: 0
• Changed suites: 1

uv-python: +4 / -0

Added:

• uv-python::discovery::tests::sequential_query_strategy_does_not_prefetch_executables
• uv-python::tests::find_all_python_installations_matches_sequential_discovery
• uv-python::tests::find_python_installation_queries_lazily
• uv-python::tests::find_python_installations_discovers_search_path_lazily

Removed: none