ProjectAnalysisHelper: make testable with minimal on-disk fixtures

[jongalloway]

ProjectAnalysisHelper is valuable but tends to drift toward integration-ish behavior.

Goal:

• Make key behavior testable via minimal, deterministic fixtures created in temp directories (small *.csproj with known contents), without requiring external SDK state beyond what CI already provides.

Acceptance:

• Add tests that create minimal on-disk projects/solutions as fixtures.
• Avoid network access and avoid depending on machine-global state.
• If any paths require real dotnet execution, gate behind existing opt-in patterns.

Parent: #247