feat(users): add support for current user OAuth applications - #3458

Changes

GitLab 19.0 added a User Applications API (/user/applications) for OAuth applications owned by the authenticated user. python-gitlab only maps the instance-wide /applications endpoint, so these could not be managed without a custom manager (see #3451).

This adds CurrentUserApplication / CurrentUserApplicationManager, exposed as gl.user.applications, following the existing gl.user.emails / gl.user.keys pattern:

• create requires name, redirect_uri, scopes; optional confidential
• list, get, delete
• update / save() with optional name and scopes

gl.auth()
app = gl.user.applications.create(
    {"name": "my_app", "redirect_uri": "http://app.url", "scopes": "read_user"}
)
app.name = "renamed"
app.save()
app.delete()

The manager is also picked up by the CLI as gitlab current-user-application {list,get,create,update,delete}.

Closes #3451

Documentation and testing

• Documentation in the matching docs section (docs/gl_objects/users.rst)
• Unit tests (tests/unit/objects/test_current_user_applications.py; create and update assert the request JSON body)

Locally: pytest tests/unit (727 passed, 3 skipped), black, isort, flake8, mypy, pylint on gitlab/, the rst pre-commit hooks, and sphinx-build -n -W all pass. I did not run the functional tests against a live GitLab 19 instance.