Add CI-backed verification for C# Fundamentals examples

[Sahithbasani]

What changed

• extracted the order, payment, and notification decision logic into reusable scenario types
• added a verify command that runs deterministic behavior checks for the executable examples
• updated the build workflow to run the verification command after the Release build
• refreshed the root and examples READMEs, including a verified .NET 10 technical blog link

Why

The repository previously built successfully, but its core example behavior was locked inside private example classes and had no executable regression path. This change makes the decision logic reusable, gives the repo a meaningful verification step, and documents the validation flow contributors should run locally.

Validation

• dotnet build src/CSharpFundamentals/CSharpFundamentals.slnx --configuration Release
• dotnet run --project src/CSharpFundamentals/CSharpFundamentals/CSharpFundamentals.csproj --configuration Release -- verify

Reviewer focus

• confirm the extracted scenario types preserve the original example behavior
• confirm the verify command covers the most valuable decision paths without adding noise
• confirm the README updates match the actual contributor workflow