refactor(marketplace): extract ref propagation helper

[WilliamK112]

Summary

• Extract the registered marketplace ref selection into a co-located helper in marketplace/resolver.py.
• Share that helper across the GitLab structured DependencyReference path and the GitHub-family canonical #ref suffix path.
• Preserve the security-adjacent guards from refactor(marketplace): extract ref-propagation helpers from resolve_marketplace_plugin #1917: explicit install/plugin refs win, main/HEAD remain implicit, and registered marketplace refs do not overwrite already-selected plugin refs.

Closes #1917

Tests

• .\.venv\Scripts\python.exe -m pytest tests\unit\marketplace\test_marketplace_resolver.py::TestMarketplaceRegisteredRefPropagation -q
• .\.venv\Scripts\python.exe -m pytest tests\unit\marketplace\test_marketplace_resolver.py -q
• .\.venv\Scripts\python.exe -m pytest tests\unit\marketplace\test_resolver_local_git.py tests\unit\marketplace\test_marketplace_commands.py -q
• .\.venv\Scripts\python.exe -m pytest tests\unit\marketplace\test_marketplace_install_integration.py::TestInstallMarketplaceGitLabMonorepoWiring -q
• .\.venv\Scripts\python.exe -m pytest tests\unit\commands\test_marketplace_plugin.py::TestResolveRef tests\unit\commands\test_marketplace_plugin.py::TestPackageAddRefResolution tests\unit\commands\test_marketplace_plugin.py::TestPackageSetRefResolution -q
• .\.venv\Scripts\python.exe -m ruff check src\apm_cli\marketplace\resolver.py
• .\.venv\Scripts\python.exe -m ruff format --check src\apm_cli\marketplace\resolver.py
• git diff --check

Note: I also tried broader marketplace pytest groups; the full tests/unit/marketplace directory and the existing TestInstallGitLabMarketplaceFullPipelineFromHttp test timed out in this Windows environment, so I did not count those as passing evidence.

[Copilot]

Pull request overview

This PR refactors marketplace ref propagation by extracting the registered-ref selection logic into a single helper in marketplace/resolver.py, then reusing it across both the GitLab structured DependencyReference path and the GitHub-family #ref suffix path while preserving the existing precedence/guard rules from #1917.

Changes:

• Introduces _select_marketplace_plugin_ref(...) to centralize ref selection precedence (version_spec > plugin ref > marketplace registered ref, excluding implicit main/HEAD).
• Replaces duplicated inline ref-selection logic in the GitLab in-marketplace dependency reference path with the new helper.
• Replaces the GitHub-family canonical #ref propagation block to use the helper and preserve non-overwrite behavior.