Add minimal permissions to dotnet workflow for improved security

[JeetKarena]

This PR updates the Dotnet workflow to include permissions: contents: read, ensuring the workflow follows the principle of least privilege and improves security.

Key changes:

• Added permissions: contents: read to the workflow file.

No functional changes to the build or test steps.

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Checklist

• Workflow file is in the correct directory and uses lower-kebab-case.
• Workflow name is "dotnet".
• Step names use sentence case.
• Workflow uses $default-branch for triggers (if applicable).
• Includes permissions: contents: read.
• Only official GitHub or ecosystem actions used.
• Comments are present and clear.