Support --output-format=json for uv python dir and gate under preview - #21858

Summary

Closes #20509.

This adds --output-format=json support to uv python dir and puts JSON output for uv python (dir and list) under preview (PreviewFeature::JsonOutput).

• Added PythonDirFormat (Text, Json) and updated uv python dir arguments and settings to accept --output-format.
• Serialized {"path": ...} using PortablePath when --output-format json is requested for uv python dir (including with --bin).
• Emitted the experimental schema warning on stderr for both uv python dir and uv python list unless --preview-features json-output (or --preview) is passed.
• Added integration snapshot tests for python dir and python list with --output-format json, --only-installed, constraints, and preview flags.

Test Plan

• Ran cargo test --package uv --test python -- python_dir
• Ran cargo test --package uv --test python -- python_list_json
• Ran cargo clippy --package uv --package uv-cli -- -D warnings
• Ran cargo fmt --all -- --check