Updates to csharp .instruction files

The current csharp instruction files have some inconsistency issues:

csharp-tests.instructions.md

• specifies xunit, but most agents interpret this as xunit v2, which is deprecated. Update to specify vunit v3
• test method name format is incompatible with examples. Should be [Given<Context>_]When<Action>_<ExpectedResult>. The Given segment is optional; the When segment may name the method under test.
• should include guidance to have one assert per test method - example has two. Assert.Equivalent in xunit.v3 also includes null check, so example of assert.notnull is not needed after bump

csharp.instructions.md

• added additional items to naming to align with csharp-tests instucti9on file and examples

Validated files with waza linter.

csharp.instructions.md

csharp-tests.instructions.md

updated files attached below

Start with csharp-tests.instructions.md and csharp.instructions.md, using the attached updated files as the intended reference. Review the testing framework guidance, test naming format, assertion guidance, and naming additions, then validate both files with the waza linter. Done means the files are consistent with the requested examples and the linter passes.