Update test asmdefs & add package testables

[IvanMurzak]

Refactor test assembly definitions across package and multiple Unity test projects: remove direct UnityEngine.TestRunner/UnityEditor.TestRunner references and nunit.framework.dll precompiled references, add optionalUnityReferences: ["TestAssemblies"], set defineConstraints to ["UNITY_INCLUDE_TESTS"] and clear precompiledReferences. Add "testables": ["com.ivanmurzak.unity.mcp.particlesystem"] to each test manifest so the package is discoverable by Unity test runners. Also slim down Unity-Package.slnx to include only the ParticleSystem projects. These changes migrate tests to use Unity's TestAssemblies workflow and make the package testable across the provided Unity versions.

[Copilot]

Pull request overview

Updates Unity test project configuration so the com.ivanmurzak.unity.mcp.particlesystem package tests can be discovered/compiled across multiple Unity versions, while simplifying test assembly references.

Changes:

• Mark com.ivanmurzak.unity.mcp.particlesystem as a testable package in the Unity test projects’ Packages/manifest.json.
• Update Unity test .asmdef files to use optionalUnityReferences: ["TestAssemblies"] instead of explicit TestRunner/NUnit references.
• Trim Unity-Package.slnx project entries to focus on the ParticleSystem assemblies.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Unity-Tests/6000.3.1f1/Packages/manifest.json	Adds testables entry for the ParticleSystem package.
Unity-Tests/6000.3.1f1/Assets/RuntimeTests/RuntimeTests.asmdef	Switches tests to optionalUnityReferences: ["TestAssemblies"] and removes explicit runner/NUnit references.
Unity-Tests/6000.3.1f1/Assets/EditorTests/EditorTests.asmdef	Same as above for editor tests.
Unity-Tests/2023.2.22f1/Packages/manifest.json	Adds testables entry for the ParticleSystem package.
Unity-Tests/2023.2.22f1/Assets/RuntimeTests/RuntimeTests.asmdef	Switches tests to optionalUnityReferences: ["TestAssemblies"] and removes explicit runner/NUnit references.
Unity-Tests/2023.2.22f1/Assets/EditorTests/EditorTests.asmdef	Same as above for editor tests.
Unity-Tests/2022.3.62f3/Packages/manifest.json	Adds testables entry for the ParticleSystem package.
Unity-Tests/2022.3.62f3/Assets/RuntimeTests/RuntimeTests.asmdef	Switches tests to optionalUnityReferences: ["TestAssemblies"] and removes explicit runner/NUnit references.
Unity-Tests/2022.3.62f3/Assets/EditorTests/EditorTests.asmdef	Same as above for editor tests.
Unity-Package/Unity-Package.slnx	Removes non-ParticleSystem project entries from the solution definition.
Unity-Package/Assets/root/Tests/Runtime/com.IvanMurzak.Unity.MCP.ParticleSystem.Tests.asmdef	Removes explicit TestRunner/NUnit references and uses optionalUnityReferences: ["TestAssemblies"].
Unity-Package/Assets/root/Tests/Editor/com.IvanMurzak.Unity.MCP.ParticleSystem.Editor.Tests.asmdef	Same as above; also adds UNITY_INCLUDE_TESTS define constraint.

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