Add conditional execution to build validation workflow

[Copilot]

Modified build validation to only run jobs when files in their respective folders change, reducing unnecessary CI execution.

Changes

• Added changes job using dorny/paths-filter@v3 to detect modified folders
• Added conditional execution to all 10 build jobs based on path filters:

  build-dotnet-samples:
    needs: changes
    if: ${{ needs.changes.outputs.dotnet == 'true' }}

• Set explicit permissions: contents: read following security best practices

Path Mappings

Each build job now triggers only when its folder changes:

• .NET/** → build-dotnet-samples
• Frontend Web with Blazor/** → build-blazor-samples
• Backend Web with Web APIs/** → build-webapi-samples
• Package Management with NuGet/** → build-nuget-samples
• Visual Studio/** → build-visualstudio-samples
• Containers/** → build-container-samples
• Artificial Intelligence and Machine Learning/** → build-ai-ml-samples
• IoT/** → build-iot-samples
• Blazor Hybrid/** → build-blazor-hybrid-samples
• .NET MAUI/** + Blazor Hybrid/** → build-maui-samples

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