Fix diagnostics package version lookup

[wunianze666-netizen]

Summary

• switch diagnostics package version lookup to importlib.metadata`n- handle PackageNotFoundError through the imported metadata module
• add a focused regression test for installed and missing packages

Why

get_package_version() currently calls importlib.metadata without importing the submodule first, so the helper can raise AttributeError before reaching its fallback path.

Testing

• compiled the updated source files locally
• ran an isolated Python verification script that stubs heavy runtime dependencies and checks both installed and missing package paths

Issue linkage: not tied to an existing tracked issue.