fix: resolve .NET E2E race conditions in cancellation and multi-client tests

[SteveSandersonMS]

Hopefully fixes two flaky .NET E2E test failures:

1. SendAndWait_Throws_OperationCanceledException_When_Token_Cancelled → Should_Create_A_Session_With_Customized_SystemMessage_Config

CI failure (Windows)

The cancellation test cancelled the token but never waited for the agent to go idle. The agent's in-flight sleep 10 request leaked into the proxy during the next test, hitting the wrong snapshot and causing a 500 error.

Fix: Wait for SessionIdleEvent after the cancellation assertion, and verify Aborted == true.

2. One_Client_Approves_Permission_And_Both_See_The_Result

CI failure (macOS)

SendAndWaitAsync returns when session1 goes idle, but client2's PermissionCompletedEvent may not have propagated yet. The immediate Assert.Contains was a race.

Fix: Set up GetNextEventOfTypeAsync<PermissionCompletedEvent>(session2) before sending, then await it before asserting.

[Copilot]

Pull request overview

Stabilizes .NET E2E tests by adding explicit event synchronization to avoid cross-test leakage after cancellation and to remove a race in multi-client permission propagation.

Changes:

• In SessionTests, adds a SessionIdleEvent waiter after canceling SendAndWaitAsync to ensure the agent becomes idle before the test ends.
• In MultiClientTests, sets up and awaits PermissionCompletedEvent on the second client before asserting both clients observed the completed permission.

Show a summary per file

File	Description
dotnet/test/SessionTests.cs	Adds waiting for session.idle after cancellation to prevent in-flight work leaking into the next test.
dotnet/test/MultiClientTests.cs	Waits for client2’s PermissionCompletedEvent before asserting, reducing a race between clients.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 2

[Copilot]

sessionIdleTask is created immediately after CreateSessionAsync(). Because session events are dispatched asynchronously, this waiter can race and capture an already-queued initial session.idle from session creation/resume rather than the post-cancellation idle you intend to wait for. To make this deterministic, set up the SessionIdleEvent waiter after you’ve observed ToolExecutionStartEvent (and before calling cts.Cancel()), so it can only match the idle corresponding to this in-flight operation.

[github-actions]

Cross-SDK Consistency Review ✅

This PR only modifies .NET test files to fix E2E race conditions — no changes to the public SDK API surface.

Changes reviewed:

• dotnet/test/SessionTests.cs — adds AbortAsync() + SessionIdleEvent waits in cancellation/timeout tests to prevent agent state from leaking between tests
• dotnet/test/MultiClientTests.cs — sets up PermissionCompletedEvent listener before sending to avoid a race in client2 assertions

These are .NET-specific test stability improvements. No features were added or modified, so there's nothing requiring equivalent changes in the Node.js, Python, or Go SDKs. No cross-SDK consistency issues identified.

Generated by SDK Consistency Review Agent for issue #1034 · ◷