handlers/internal: fix db.DB pollution in registry and scheduler test helpers

Five more test helpers have the same setupTestDB bug (save db.DB but
don't restore on teardown). go test -race runs tests in parallel; when
test A sets db.DB = mockA and test B sets db.DB = mockB, if A runs
first and cleanup closes mockA, B then runs with db.DB pointing at a
closed mock.

Fixed files:
- internal/registry/liveness_test.go    setupLivenessTestDB
- internal/registry/hibernation_test.go  setupHibernationMock
- internal/registry/access_test.go      setupMockDB
- internal/registry/healthsweep_test.go  setupTestDB
- internal/scheduler/scheduler_test.go   setupTestDB

All now follow: prevDB := db.DB; db.DB = mockDB;
t.Cleanup(func() { mockDB.Close(); db.DB = prevDB })

Total files fixed for mc#975: 8 files, ~20 test helper functions across
the workspace-server. Together with the CI fix to remove the
PHASE3_MASKED workaround, this should make CI/Platform (Go) stable.

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

workspace-server/internal/registry/access_test.go

@@ -14,8 +14,9 @@ func setupMockDB(t *testing.T) sqlmock.Sqlmock {
 	if err != nil {
 		t.Fatalf("sqlmock: %v", err)
 	}
+	prevDB := db.DB
 	db.DB = mockDB
-	t.Cleanup(func() { mockDB.Close() })
+	t.Cleanup(func() { mockDB.Close(); db.DB = prevDB })
 	return mock
 }
 

workspace-server/internal/registry/healthsweep_test.go

@@ -31,8 +31,9 @@ func setupTestDB(t *testing.T) sqlmock.Sqlmock {
 	if err != nil {
 		t.Fatalf("failed to create sqlmock: %v", err)
 	}
+	prevDB := db.DB
 	db.DB = mockDB
-	t.Cleanup(func() { mockDB.Close() })
+	t.Cleanup(func() { mockDB.Close(); db.DB = prevDB })
 	return mock
 }
 

workspace-server/internal/registry/hibernation_test.go

@@ -17,8 +17,9 @@ func setupHibernationMock(t *testing.T) sqlmock.Sqlmock {
 	if err != nil {
 		t.Fatalf("sqlmock.New: %v", err)
 	}
+	prevDB := db.DB
 	db.DB = mockDB
-	t.Cleanup(func() { mockDB.Close() })
+	t.Cleanup(func() { mockDB.Close(); db.DB = prevDB })
 	return mock
 }
 

workspace-server/internal/registry/liveness_test.go

@@ -18,8 +18,9 @@ func setupLivenessTestDB(t *testing.T) sqlmock.Sqlmock {
 	if err != nil {
 		t.Fatalf("failed to create sqlmock: %v", err)
 	}
+	prevDB := db.DB
 	db.DB = mockDB
-	t.Cleanup(func() { mockDB.Close() })
+	t.Cleanup(func() { mockDB.Close(); db.DB = prevDB })
 	return mock
 }
 

workspace-server/internal/scheduler/scheduler_test.go

@@ -24,8 +24,9 @@ func setupTestDB(t *testing.T) sqlmock.Sqlmock {
 	if err != nil {
 		t.Fatalf("failed to create sqlmock: %v", err)
 	}
+	prevDB := db.DB
 	db.DB = mockDB
-	t.Cleanup(func() { mockDB.Close() })
+	t.Cleanup(func() { mockDB.Close(); db.DB = prevDB })
 	return mock
 }