.NET: Fix .NET Copilot integration tests for SDK v1.0.0 (#6424)

* Fix .NET Copilot integration tests for SDK v1.0.0

- Remove hard-skip in favor of runtime Assert.Skip when COPILOT_GITHUB_TOKEN is not set
- Add [Trait("Category", "Integration")] for CI filtering
- Fix FunctionTool test: use explicit SessionConfig with Tools, OnPermissionRequest, and SystemMessage
- Mark RemoteMcp test as IntegrationDisabled (requires OAuth flow)
- Create explicit sessions in all tests and delete after each (cleanup)
- Remove unused System.Diagnostics import
- Simplify SkipIfCopilotNotConfigured to only check env var

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address review: use try/finally for session cleanup, IsNullOrWhiteSpace

- Wrap act/assert in try/finally so sessions are always deleted even on failure
- Use IsNullOrWhiteSpace instead of IsNullOrEmpty for token check

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add COPILOT_GITHUB_TOKEN to .NET integration test workflow

The Copilot SDK runtime reads this env var directly for authentication.
No Node.js/npm install needed - the SDK downloads the CLI binary at build time.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/dotnet-integration-tests.yml

@@ -88,6 +88,7 @@ jobs:
         env:
           COSMOSDB_ENDPOINT: https://localhost:8081
           COSMOSDB_KEY: C2y6yDjf5/R+ob0N8A7Cgv30VRDJIWEHLM+4QDU5DE2nQ9nDuVTqobD4b8mGGyPMbIZnqyMsEcaGQy67XIw/Jw==
+          COPILOT_GITHUB_TOKEN: ${{ secrets.COPILOT_GITHUB_TOKEN }}
           OpenAI__ApiKey: ${{ secrets.OPENAI__APIKEY }}
           OpenAI__ChatModelId: ${{ vars.OPENAI__CHATMODELID }}
           OpenAI__ChatReasoningModelId: ${{ vars.OPENAI__CHATREASONINGMODELID }}