Update Copilot instructions for MTP

[Youssef1313]

Motivation and Context

How Has This Been Tested?

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[halter73]

I did merge #1011, so we'll probably want to include the reversion of that in this PR. Additionally, I think we should call out --project is necessary for specifying specific test projects.

Suggested change

	- Valid test filter switches include: `--filter-class`, `--filter-not-class`, `--filter-method`, `--filter-not-method`, `--filter-namespace`, `--filter-not-namespace`, `--filter-not-trait`, `--filter-trait`. The switches `--filter-class` and `--filter-method` expect fully qualified names, unless a filter is used as a prefix like `--filter-class "*.SomeClassName"` or `--filter-method "*.SomeMethodName"`. These switches can be repeated to run tests on multiple classes or methods at once, e.g., `--filter-method "*.SomeMethodName1" --filter-method "*.SomeMethodName2"`.
	- Valid test filter switches include: `--filter-class`, `--filter-not-class`, `--filter-method`, `--filter-not-method`, `--filter-namespace`, `--filter-not-namespace`, `--filter-not-trait`, `--filter-trait`. The switches `--filter-class` and `--filter-method` expect fully qualified names, unless a filter is used as a prefix like `--filter-class "*.SomeClassName"` or `--filter-method "*.SomeMethodName"`. These switches can be repeated to run tests on multiple classes or methods at once, e.g., `--filter-method "*.SomeMethodName1" --filter-method "*.SomeMethodName2"`.
	- To specify a specific test project to run, you must always use the `--project` switch. For example, `dotnet test --project <path to test project directory or .csproj file> --filter-not-trait 'Execution=Manual'`.

I'm not sure if there are more incompatibilities, but it would be better to avoid any breaking changes to the dotnet test cli interface to begin with.

[halter73]

I'm closing this since we reverted the update to xunit.v3.mtp-v2. We can update the copilot instructions once we try it again, but I hope that we can avoid the breaking changes to the cli interface altogether.