Apply review follow-up fixes

[MasayukiOzawa]

Summary

• Reuse ShowplanParser's existing attribute helper for root metadata.
• Remove the now-unneeded ShowplanXmlElement helper.
• Rename the diagnostics stub constructor parameter to avoid ambiguity with the field.

Testing

• dotnet test .\MSSQLPlanViewer.sln -p:BaseOutputPath=.\artifacts\test-bin\

[Copilot]

Pull request overview

This PR applies follow-up refactors in the Core parsing layer and test stubs by consolidating XML attribute access into ShowplanParser’s existing helpers, removing an unused helper type, and clarifying a stub constructor parameter name to avoid ambiguity.

Changes:

• Replaced root metadata attribute reads in ShowplanParser to use its internal GetAttribute helper.
• Removed the now-unused ShowplanXmlElement helper type from Core parsing.
• Renamed the diagnostics stub constructor parameter in tests to avoid confusion with the backing field.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/MSSQLPlanViewer.Core.Tests/PlanWorkspaceServiceTests.cs	Renames the stub diagnostics constructor parameter to avoid ambiguity with the diagnostics field.
src/MSSQLPlanViewer.Core/Parsing/ShowplanXmlElement.cs	Deletes an unused XML helper now replaced by ShowplanParser’s internal helpers.
src/MSSQLPlanViewer.Core/Parsing/ShowplanParser.cs	Reuses the parser’s existing GetAttribute helper for root Version/Build metadata extraction.