CI: AgentToolTrackingControllerTests delivery-barrier test can drop observer event on shard 2

[baron]

Impact

CI app shard 2 can fail on an Agent Mode tool-tracking concurrency test, blocking otherwise unrelated PR validation noise and making it hard to distinguish product regressions from test instability.

Observed behavior

On PR #388, exact head a1b5fbe8, Build and Test (app shard 2) failed twice on the same test after a one-time rerun:

• AgentToolTrackingControllerTests/testConcurrentRawUnregisterAndStopJoinCapturedDeliveryBarrier
• Failure location: Tests/RepoPromptTests/AgentMode/AgentToolTrackingControllerTests.swift:227
• Assertion: expected firedCount == 1, observed firedCount == 0
• Distinctive log excerpt:

[ClaudeToolTracking] MCP observer call skipped no event observers

The failed workflow was the PR CI run for head a1b5fbe8; all other CI jobs for that head completed successfully.

Expected behavior

The test should deterministically capture the intended delivery-barrier event, or the contract should be rewritten so the ordering it asserts is guaranteed by the implementation.

Evidence suggesting this is unrelated to PR #388

PR #388 changes MCP agent-run/worktree inheritance and startup-pending wait snapshot behavior. It does not touch:

• Tests/RepoPromptTests/AgentMode/AgentToolTrackingControllerTests.swift
• AgentToolTrackingController
• the tool-tracking observer registration/unregistration implementation path implicated by the log excerpt

A local focused run of the failing test passed:

make dev-test FILTER='AgentToolTrackingControllerTests/testConcurrentRawUnregisterAndStopJoinCapturedDeliveryBarrier'

That combination points to a timing/order-sensitive CI flake or latent concurrency issue rather than a regression in the PR's changed code path.

Reproduction notes

1. Run PR CI app shard 2 on a head that includes the current test suite.
2. Observe whether AgentToolTrackingControllerTests/testConcurrentRawUnregisterAndStopJoinCapturedDeliveryBarrier fails with firedCount == 0.
3. Compare against a focused local run of the same test.

Frequency from the observed PR: 2/2 hosted shard-2 attempts failed after the initial failure and one rerun; focused local run passed once.

Acceptance criteria

• The test no longer fails because observer delivery sees an empty observer map under the captured-delivery-barrier scenario, or the test is adjusted to assert only behavior guaranteed by the production contract.
• A focused run of AgentToolTrackingControllerTests/testConcurrentRawUnregisterAndStopJoinCapturedDeliveryBarrier passes.
• CI shard 2 can pass without requiring repeated reruns for this specific test.