ci: split PR checks into explicit job groups

[rian-be]

Summary

This PR keeps the PR checks readable by splitting examples out of the main CI workflow and leaving build/test verification in pr-check.yml.

Highlights

CI workflow

• Keep pr-check.yml focused on build and test validation.
• Move example validation into a separate examples-check.yml workflow.
• Preserve explicit jobs for core examples, governance examples, and the Redis example.
• Keep job names specific so failures point at the affected surface.

Governance tests

• Add a dedicated approval-request helper for the target-mapping scenario.
• Align the approval workflow test with the request shape it validates.

Why

The previous workflow was correct but still too tall in the GitHub Actions UI. Splitting example validation into a second workflow keeps the checks easier to scan while preserving explicit coverage for each repository surface.

Checks

• dotnet build ModularityKit.Mutator.slnx -c Release
• dotnet test Tests/ModularityKit.Mutator.Governance.Tests/ModularityKit.Mutator.Governance.Tests.csproj -c Release
• dotnet test Tests/ModularityKit.Mutator.Tests/ModularityKit.Mutator.Tests.csproj -c Release --no-build --no-restore
• dotnet test Tests/ModularityKit.Mutator.Governance.Redis.Tests/ModularityKit.Mutator.Governance.Redis.Tests.csproj -c Release --no-build --no-restore
• dotnet run --project Examples/Core/BillingQuotas/BillingQuotas.csproj -c Release --no-build --no-restore
• dotnet run --project Examples/Governance/RequestLifecycle/RequestLifecycle.csproj -c Release --no-build --no-restore

Closes

• Closes [Task]: Add CI matrix for examples and test packages #51