Add matrix testing for package installation on Windows/Linux with .NET 8/9/10

[Copilot]

The NuGet package was not being tested after build, creating risk that the packaged pandoc tool might not execute correctly on target platforms.

Changes

• Test Job: Added matrix testing across 6 configurations (Windows/Linux × .NET 8/9/10)

  • Downloads build artifacts
  • Installs tool via dotnet tool install --global
  • Validates execution with pandoc --version and pandoc --help

• Security: Added explicit permissions: contents: read blocks to both jobs

• Documentation: Updated AGENTS.md and CONTRIBUTING.md with CI testing details

• Build Runner: Changed from ubuntu-24.04 to ubuntu-latest for consistency

The test job depends on build completion and will fail if the tool cannot be installed or executed on any platform/runtime combination.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Feature]: Add package tests</issue_title>
<issue_description>### Feature Description

After building the package, it should be tested on both Windows and Linux to ensure the packaged Doxygen tool executes correctly.

Problem Statement

The package isn't tested as part of being built or released.

Proposed Solution

The build.yaml file should be updated to:

1. Run the existing build job, but move it to 'ubuntu-latest'
2. Add a test job which matrix-tests on Windows and Linux as well as DotNet 8, 9, and 10 by downloading the NuGet package created in the build job, installing it, and then running the pandoc tool with a simple command line such as asking for help and then checking the tool ran without error.

The CI testing information should be updated in the AGENTS.md and CONTRIBUTING.md files.

Alternatives Considered

No response

Additional Context

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Feature]: Add package tests #12

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