Refactored direct_url.json parsing to eliminate complexity violations and
add comprehensive type validation matching pip's strict validation model.

Extracted parse_direct_url_json into helper functions to reduce complexity:
- _load_and_validate_json: JSON loading and structure validation
- _validate_required_fields: Type validation for url/subdirectory
- _parse_info_block: Info block selection and validation

Enhanced VCS detection to handle repository subdirectories using git
rev-parse --show-toplevel, allowing editable packages in subdirectories.
Added URL normalization for SCP-style (git@host:path), git://, and local
paths. Added subdirectory fragment support for monorepo packages.

Fixed file:// URL conversion to match pip's implementation exactly,
handling localhost URLs, UNC paths on Windows, and Windows drive letter
corrections. Rejects non-local file URIs on non-Windows platforms.

Added comprehensive type validation for all direct_url.json fields:
- Validates url and subdirectory are strings
- Validates all info blocks are dicts
- Validates vcs_info fields (vcs, commit_id, requested_revision) are strings
- Validates archive_info.hash matches algo=value pattern
- Validates dir_info.editable is boolean

Changed DirectUrlValidationError to inherit from ValueError instead of
Exception, following Python conventions for value validation errors.

All 84 parser tests pass with 95.63% coverage maintained.

Fixed git:// URLs being incorrectly prefixed with git+ (should be git://... not
git+git://...). Now only adds git+ prefix when URL doesn't already start with git:,
matching pip's behavior per versioncontrol.py:262.

Fixed url_to_path double-decoding issue where unquote was called before url2pathname,
causing file:///x%2520y to become x y instead of x%20y like pip. Removed unquote since
url2pathname handles percent-decoding.

Refactored _get_git_requirement to eliminate complexity violations by extracting helper
functions: _get_repo_root, _get_remote_url, _get_commit_id, _build_vcs_requirement.

Fixed hash validation to accept any algo=value format (including md5, uppercase) to
match pip's lenient validation instead of strict regex.

Added platform-specific pragma comments for Windows-only code paths (UNC paths, drive
letter handling) that can't be tested on non-Windows platforms.

Added PLR2004 to global ruff ignore since magic number checks are disabled project-wide.
Added pytest-subprocess>=1.5 to test dependencies for declarative subprocess mocking.

All 99 tests pass with 100% diff coverage.

- Redact token-only credentials (no colon) to ****@host
- Always prefix git URLs with git+ (including git:// protocol)
- Skip subdirectory fragment for svn and bzr backends
- Convert hg/bzr local-path remotes to file:// URLs
- Return COMMAND_NOT_FOUND instead of NO_REMOTE for missing
  VCS binaries

Resolve relative-path remotes (e.g. ../repo.git) against repo_root
in _normalize_git_url, matching pip's behavior of treating any
existing path as local.

Pip's DirectUrl._remove_auth_from_netloc preserves git@ only when
vcs_info.vcs == "git", strips all other auth entirely (no ****@),
and preserves ${VAR} patterns. Align our implementation by passing
info to _redact_url.

Use re.VERBOSE with named groups for all regex patterns. Tighten
env-var detection to match pip's ENV_VAR_RE (only ${VAR} or
${VAR}:${VAR}), preventing credential leak in mixed patterns
like ${TOKEN}:pass@.

- Make cross-module imports public with __all__ re-exports
- Compile regex as module-level Final[re.Pattern[str]]
- Support hg shared repos (.hg as file) via dir_only param
- Format tests with setup/run/assert blank line separation
- Rename test helpers and test names to reference public API

uv's direct_url.json has commit_id as Optional<String>. Handle
gracefully instead of rejecting the entire direct_url.json.