Fix MCP E2E send wait race

[stephentoub]

Fixes a race in the MCP and custom-agent configuration E2E coverage where the test sent a prompt and only then waited for the final assistant message. If the replayed turn completed quickly, the ephemeral session.idle event could be missed and the test would time out.

This switches those simple verification prompts to SendAndWaitAsync, which registers its event handler before sending.

Tests:

• dotnet build dotnet\test\GitHub.Copilot.SDK.Test.csproj -f net8.0 --no-restore --nologo

Generated by Copilot

[Copilot]

Pull request overview

This PR updates a couple of MCP/custom-agent E2E test interactions to avoid a race where the test could miss the transient session.idle event if the replayed turn completes very quickly. It does so by using SendAndWaitAsync, which registers the event handler before sending the prompt, making these verification steps more reliable within the broader .NET E2E suite.

Changes:

• Replace SendAsync(...) + TestHelper.GetFinalAssistantMessageAsync(...) with SendAndWaitAsync(...) in two “simple interaction” verification steps.
• Ensure the final assistant message is awaited via the session’s built-in idle-waiting mechanism to prevent timeouts caused by missed idle events.

Show a summary per file

File	Description
dotnet/test/E2E/SessionMcpAndAgentConfigE2ETests.cs	Switches two simple verification prompts to SendAndWaitAsync to eliminate an idle-event timing race in E2E tests.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0