Fix Maven version parsing for non-numeric versions

[Vadim-Zenin]

Problem

MavenContentMixin.group_artifact_version_filename() decides whether a path
contains a version directory by matching the candidate segment against a
numeric-anchored regex (re.match(r"\d+...")). Maven versions are free-form,
so any version not starting with a digit — RELEASE, master-SNAPSHOT,
JIRA-tagged INT-7144.1.0.2, etc. — is misclassified: the version segment is
taken as the artifactId, version becomes None, and the upload fails with
a NOT NULL violation → HTTP 500 on PUT /pulp/maven/<distribution>/<path>.

Fixes #

Fix

The content type is already known upstream
(MavenRemote.get_remote_artifact_content_type() routes .xml* to
MavenMetadata, everything else to MavenArtifact). A binary artifact in
standard Maven layout always has a version directory, so the regex guess is
unnecessary and wrong there. This PR adds a version_present argument:

• MavenArtifact passes version_present=True — version directory is always
  present, no guessing.
• MavenMetadata and any other caller keep the existing best-effort detection
  (default version_present=None), so artifact-level maven-metadata.xml
  (which has no version directory) is unchanged.

No behavior change for numeric versions or for metadata; non-numeric artifact
versions now parse correctly.

Tests

Added pulp_maven/tests/unit/test_path_parsing.py covering numeric and
non-numeric versions plus the artifact-level-metadata (no-version) case.

Changelog

CHANGES/.bugfix added.