fix(handlers): replace time.Sleep with explicit async drain in 4 tests

Issue #1264 — CI/Platform Go tests flake under parallel load.

The 4 tests that were failing used time.Sleep(N) to wait for
goroutines launched by goAsync() to complete before assertions
ran. Under CI parallelism, goroutines from prior tests could
still be writing to the next test's sqlmock, and the fixed
sleep durations (50–200ms) were insufficient under load.

Fix: replace each time.Sleep with handler.waitAsyncForTest(), which
calls h.asyncWG.Wait() and returns only when all goroutines started
by this handler have terminated. This is deterministic regardless
of parallelism or machine speed.

Tests changed:
- TestProxyA2A_Upstream502_TriggersContainerDeadCheck
- TestProxyA2A_Upstream502_AliveAgent_PropagatesAsIs
- TestGracefulPreRestart_URLResolutionError
- TestRestartWorkspaceAuto_RoutesToDockerWhenOnlyDocker

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace-server/internal/handlers/a2a_proxy_test.go

@@ -294,8 +294,7 @@ func TestProxyA2A_Upstream502_TriggersContainerDeadCheck(t *testing.T) {
 	c.Request.Header.Set("Content-Type", "application/json")
 
 	handler.ProxyA2A(c)
-
-	time.Sleep(80 * time.Millisecond)
+	handler.waitAsyncForTest()
 
 	// Caller sees a structured 503 (NOT the upstream 502 which CF would mask).
 	if w.Code != http.StatusServiceUnavailable {
@@ -350,7 +349,7 @@ func TestProxyA2A_Upstream502_AliveAgent_PropagatesAsIs(t *testing.T) {
 	c.Request.Header.Set("Content-Type", "application/json")
 
 	handler.ProxyA2A(c)
-	time.Sleep(50 * time.Millisecond)
+	handler.waitAsyncForTest()
 
 	if w.Code != http.StatusBadGateway {
 		t.Fatalf("alive agent 502 should propagate as 502; got %d: %s", w.Code, w.Body.String())

workspace-server/internal/handlers/restart_signals_test.go

@@ -273,7 +273,7 @@ func TestGracefulPreRestart_URLResolutionError(t *testing.T) {
 	}
 
 	hWrapper.gracefulPreRestart(context.Background(), "ws-url-err-111")
-	time.Sleep(200 * time.Millisecond)
+	hWrapper.waitAsyncForTest()
 	// No panic or error expected — proceeds with stop as documented
 }
 

workspace-server/internal/handlers/workspace_provision_auto_test.go

@@ -686,7 +686,7 @@ func TestRestartWorkspaceAuto_RoutesToDockerWhenOnlyDocker(t *testing.T) {
 	// recovered by logProvisionPanic. Without this wait, the goroutine
 	// outlives the test and writes to a sqlmock that the NEXT test
 	// owns, causing a `was not expected` race.
-	time.Sleep(200 * time.Millisecond)
+	h.waitAsyncForTest()
 
 	// Stop call is synchronous on the Docker leg.
 	if len(stub.stopped) == 0 || stub.stopped[0] != wsID {