Bug: flaky test — AgentChatFactoryTests.RestoreSubAgentsAsync_ChildTerminalTaskFaults_ParentInitializeCompletes (SubAgents.Count race)

[JoshuaRowePhantom]

Unrelated flaky test — observed during checkin for #1458

Failing test: Phantom.Workspaces.Llm.Tests.AgentChatFactoryTests.RestoreSubAgentsAsync_ChildTerminalTaskFaults_ParentInitializeCompletes

Failure message:

Assert.Equal() Failure: Values differ
Expected: 2
Actual:   0
   at Phantom.Workspaces.Llm.Tests.AgentChatFactoryTests.RestoreSubAgentsAsync_ChildTerminalTaskFaults_ParentInitializeCompletes()
   in Phantom.Workspaces.Llm.Core.Tests/AgentChatFactoryTests.cs:line 726

No dump — this is an assertion failure, not a hang/crash.

Reproduction characterization

• Failed once during the full fast suite (6290 tests, under heavy parallel scheduler load).
• Passed 6/6 times in isolation (dotnet test --filter FullyQualifiedName~RestoreSubAgentsAsync_ChildTerminalTaskFaults_ParentInitializeCompletes).
• Classic load-dependent race.

Root cause hypothesis

In Phantom.Workspaces.Llm.Core/AgentChat.cs RestoreSubAgentsAsync (lines ~1382–1408), each restored child is added to the observable subAgentItems collection via a fire-and-forget task on the foreground scheduler:

_ = Task.Factory.StartNew(
    () => this.subAgentItems.Add(stub),
    CancellationToken.None,
    TaskCreationOptions.DenyChildAttach,
    this.foregroundScheduler);

The test synchronizes on WaitForRestoredSubAgentsMarkedTerminalAsync(), which only awaits restoredSubAgentTerminalTasks (the MarkRestoredSubAgentTerminalAsync tasks). It does not await the foreground-scheduler subAgentItems.Add tasks. Under full-suite scheduler contention those Add tasks can still be pending when the test reads lease.AgentChat.SubAgents.Count, so the assertion observes 0 instead of 2.

Proposed solution

Make the completion of the subAgentItems.Add foreground tasks observable and include them in the wait barrier:

1. Track the Task.Factory.StartNew(... subAgentItems.Add ...) tasks in a list (e.g. restoredSubAgentAddTasks) under a lock, mirroring restoredSubAgentTerminalTasks.
2. Have WaitForRestoredSubAgentsMarkedTerminalAsync() (or a new WaitForRestoredSubAgentsAddedAsync()) Task.WhenAll those Add tasks in addition to the terminal tasks.
3. Update the test to await that combined barrier before asserting SubAgents.Count == 2.

Alternatively, flush the foreground scheduler by scheduling a completion sentinel on foregroundScheduler and awaiting it before the assertion.

This is purely a test/synchronization gap in the sub-agent restore path; the production behavior (eventual population on the foreground scheduler) is correct.