[MSTest] Do not use var when type is not explicit

[Evangelink]

Summary

Apply feedback about usage of var in MSTest docs.

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Internal previews

📄 File	🔗 Preview link
docs/core/testing/unit-testing-mstest-writing-tests-lifecycle.md	MSTest lifecycle
docs/core/testing/unit-testing-mstest-writing-tests.md	Write tests with MSTest

[Copilot]

Pull request overview

This PR updates MSTest documentation and snippets to avoid var when the assigned type isn’t explicit from the right-hand side, improving clarity and aligning the examples with the intended guidance.

Changes:

• Replaced var with int for Calculator.Add(...) results in MSTest docs.
• Replaced var with HttpResponseMessage for HttpClient.GetAsync(...) results in lifecycle examples.
• Replaced var with bool for IsPrime(...) results in the MSTest sample test snippet.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
docs/core/testing/unit-testing-mstest-writing-tests.md	Makes result types explicit in introductory MSTest examples where the RHS doesn’t show the type.
docs/core/testing/unit-testing-mstest-writing-tests-lifecycle.md	Makes HTTP response and calculator result types explicit in lifecycle examples.
docs/core/testing/snippets/unit-testing-using-mstest/csharp/PrimeService.Tests/PrimeService_IsPrimeShould.cs	Makes IsPrime result type explicit in the MSTest sample snippet.