Fix flaky macOS test: isolate agent ID to prevent parallel test class interference

WaitForCompletion_PreviouslyBusyPidDisappeared used shared "default" agent ID,
which raced with PipeDiscoveryServiceTests calling ConsumeKnownBusyPids on the
same singleton. Use AllocateSubAgentId() for test isolation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: yotsuda

Tests/Unit/Proxy/PowerShellToolsTests.cs

@@ -566,9 +566,13 @@ public async Task WaitForCompletion_PreviouslyBusyPidDisappeared_ReturnsClosedMe
         var sessionManager = ConsoleSessionManager.Instance;
         const int testPid = 88802;
 
+        // Use a unique agent ID to avoid race with parallel test classes (e.g. PipeDiscoveryServiceTests)
+        // that also call ConsumeKnownBusyPids on the singleton with "default" agent ID
+        var isolatedAgentId = sessionManager.AllocateSubAgentId();
+
         sessionManager.TryAssignNameToPid(testPid);
         var expectedDisplayName = sessionManager.GetConsoleDisplayName(testPid);
-        sessionManager.MarkPipeBusy(TestAgentId, testPid);
+        sessionManager.MarkPipeBusy(isolatedAgentId, testPid);
 
         _mockPipeDiscoveryService
             .Setup(s => s.CollectAllCachedOutputsAsync(It.IsAny<string>(), It.IsAny<string?>(), It.IsAny<CancellationToken>()))
@@ -579,7 +583,7 @@ public async Task WaitForCompletion_PreviouslyBusyPidDisappeared_ReturnsClosedMe
             _mockPowerShellService.Object,
             _mockPipeDiscoveryService.Object,
             timeout_seconds: 1,
-            agent_id: TestAgentId);
+            agent_id: isolatedAgentId);
 
         // Assert: console display name (not raw pipe name) in closed message
         Assert.Contains(expectedDisplayName, result);