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

Validation

• uv run --frozen --extra dev pytest tests/unit/marketplace/test_marketplace_resolver.py -q -n0 (127 passed)
• uv run --frozen --extra dev pytest tests/unit/marketplace/test_resolver_local_git.py tests/unit/marketplace/test_marketplace_commands.py tests/unit/marketplace/test_marketplace_install_integration.py::TestInstallMarketplaceGitLabMonorepoWiring 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 -n0 (73 passed)
• uv run --frozen --extra dev ruff check src/apm_cli/marketplace/resolver.py
• uv run --frozen --extra dev ruff format --check src/apm_cli/marketplace/resolver.py
• BASE_REF=origin/main bash tests/spec_conformance/mode_b_detector.sh
• git diff --check origin/main...HEAD

[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.

[WilliamK112]

Rebased the single refactor commit onto the rewritten current main history and preserved the newer auth-scheme handling while resolving the helper conflict. Validation is green: 200 marketplace/ref tests pass; Ruff, the Mode B gate, and git diff --check are clean.