docs: prohibit InternalsVisibleTo in .NET test guidance (#804)

* Initial plan

* Add .NET InternalsVisibleTo note to copilot instructions

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

@@ -25,6 +25,7 @@
   - Python: `cd python && uv pip install -e ".[dev]"` → `uv run pytest` (E2E tests use the test harness)
   - Go: `cd go && go test ./...`
   - .NET: `cd dotnet && dotnet test test/GitHub.Copilot.SDK.Test.csproj`
+  - **.NET testing note:** Never add `InternalsVisibleTo` to any project file when writing tests. Tests must only access public APIs.
 
 ## Testing & E2E tips ⚙️