handlers: restore db.DB after each test to fix CI/Platform (Go) race failures

mc#975 root cause: TestListDelegationsFromLedger_* and
TestListDelegationsFromActivityLogs_* assign db.DB = mockDB then defer
mockDB.Close(), but never save/restore the previous db.DB value. With
go test -race (parallel execution), any test running after one of these
13 tests sees db.DB pointing at a closed sqlmock and fails.

Fix: save prevDB := db.DB before assignment, then t.Cleanup(func() {
mockDB.Close(); db.DB = prevDB }) — the same pattern already used by
setupTestDB for the SSRF/restore path.

Also fix setupTestDB in handlers_test.go: it called t.Cleanup(func()
{ mockDB.Close() }) but left db.DB pointing at the closed mock; now it
also restores prevDB.

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

workspace-server/internal/handlers/delegation_list_test.go

@@ -145,6 +145,52 @@ func TestListDelegationsFromLedger_MultipleRows(t *testing.T) {
 	}
 }
 
+=======
+func TestListDelegationsFromLedger_NullsOmitted(t *testing.T) {
+	// last_heartbeat, deadline, result_preview, error_detail are all NULL.
+	// Handler must not panic and must omit those keys from the map.
+	mockDB, mock, err := sqlmock.New()
+	if err != nil {
+		t.Fatalf("failed to create sqlmock: %v", err)
+	}
+	prevDB := db.DB
+	db.DB = mockDB
+	t.Cleanup(func() { mockDB.Close(); db.DB = prevDB })
+
+	now := time.Now()
+	rows := sqlmock.NewRows([]string{}).
+		AddRow("del-1", "ws-1", "ws-2", "task", "queued", nil, nil, nil, nil, now, now)
+	mock.ExpectQuery("SELECT .+ FROM delegations").
+		WithArgs("ws-1").
+		WillReturnRows(rows)
+
+	broadcaster := newTestBroadcaster()
+	wh := NewWorkspaceHandler(broadcaster, nil, "http://localhost:8080", t.TempDir())
+	dh := NewDelegationHandler(wh, broadcaster)
+
+	got := dh.listDelegationsFromLedger(context.Background(), "ws-1")
+	if len(got) != 1 {
+		t.Fatalf("expected 1 entry, got %d", len(got))
+	}
+	e := got[0]
+	if _, ok := e["last_heartbeat"]; ok {
+		t.Error("last_heartbeat should be absent when NULL")
+	}
+	if _, ok := e["deadline"]; ok {
+		t.Error("deadline should be absent when NULL")
+	}
+	if _, ok := e["response_preview"]; ok {
+		t.Error("response_preview should be absent when NULL result_preview")
+	}
+	if _, ok := e["error"]; ok {
+		t.Error("error should be absent when NULL error_detail")
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("sqlmock expectations: %v", err)
+	}
+}
+
+>>>>>>> 5531b471 (handlers: restore db.DB after each test to fix CI/Platform (Go) race failures)
 func TestListDelegationsFromLedger_QueryError(t *testing.T) {
 	// Query failure returns nil — graceful fallback, no panic.
 	mockDB, mock, err := sqlmock.New()
@@ -439,6 +485,7 @@ func TestListDelegationsFromActivityLogs_RowsErr(t *testing.T) {
 	}
 }
 
+<<<<<<< HEAD
 // TestListDelegationsFromActivityLogs_ScanErrorSkipped is removed.
 //
 // Same reason as TestListDelegationsFromLedger_ScanError: Go 1.25 causes

workspace-server/internal/handlers/handlers_test.go

@@ -29,6 +29,11 @@ func init() {
 // setupTestDB creates a sqlmock DB and assigns it to the global db.DB.
 // It also disables the SSRF URL check so that httptest.NewServer loopback
 // URLs and fake hostnames (*.example) used in tests don't trigger rejections.
+//
+// IMPORTANT: db.DB is saved before assignment and restored via t.Cleanup so
+// that tests running after this one are not polluted by a closed mock.
+// This is the single root cause of the systemic CI/Platform (Go) failures on
+// main HEAD 8026f020 (mc#975).
 func setupTestDB(t *testing.T) sqlmock.Sqlmock {
 	t.Helper()
 	mockDB, mock, err := sqlmock.New()