Improve best practices for MSTest

[Evangelink]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• My contribution adds a new instruction, prompt, agent, or skill file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, or skill with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

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Description

Improves best practices for MSTest to take into consideration new features, analyzers recommendations and advanced guidelines.

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New collection file.
• New skill file.
• Update to existing instruction, prompt, agent, collection or skill.
• Other (please specify):

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Additional Notes

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[Copilot]

Pull request overview

This pull request significantly updates the MSTest best practices prompt to reflect modern MSTest 3.x/4.x features, APIs, and conventions. The update transforms a basic guide into a comprehensive reference for contemporary MSTest development.

Changes:

• Updated to focus on MSTest 3.x/4.x with modern APIs including new assertion methods, conditional execution, retry functionality, and work item traceability
• Added extensive documentation on modern assertion APIs (Assert.Throws, Assert.Contains, comparison assertions, etc.)
• Expanded test lifecycle guidance with constructor preference over TestInitialize and detailed execution order
• Enhanced data-driven testing section with ValueTuple and TestDataRow patterns
• Added comprehensive TestContext coverage including cancellation token support
• Included advanced features like OSCondition, CICondition, Retry attribute, and work item linking
• Added "Common Mistakes to Avoid" section with concrete examples

Comments suppressed due to low confidence (2)

prompts/csharp-mstest.prompt.md:448

• In the "Common Mistakes" section, the example shows var handler = Assert.IsInstanceOfType<MyHandler>(result); as the correct approach. However, this is inconsistent with the earlier Type Assertions section (line 170) which suggests the method has an out parameter. You should ensure the API usage is consistent throughout the document and matches the actual MSTest API.

var handler = Assert.IsInstanceOfType<MyHandler>(result);

prompts/csharp-mstest.prompt.md:5

• The prompt file is missing the model field in the front matter. According to the coding guidelines, you should strongly encourage the use of model to specify the model that the prompt is optimized for. Consider adding a line like model: 'gpt-4.1' or model: 'Claude Sonnet 4' to indicate which AI model this prompt was designed for.

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agent: 'agent'
tools: ['changes', 'search/codebase', 'edit/editFiles', 'problems', 'search']
description: 'Get best practices for MSTest 3.x/4.x unit testing, including modern assertion APIs and data-driven tests'
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