[Synthetics] Fix params sharing !! (elastic#219675)

## Summary

Fix params sharing, added a unit test for the change !!

Author: shahzad31

x-pack/solutions/observability/plugins/synthetics/server/synthetics_service/synthetics_service.test.ts

@@ -450,6 +450,7 @@ describe('SyntheticsService', () => {
         },
       });
     });
+
     it('returns the space limited params', async () => {
       const { service } = getMockedService();
       jest.spyOn(service, 'getSyntheticsParams').mockRestore();
@@ -471,6 +472,44 @@ describe('SyntheticsService', () => {
         },
       });
     });
+
+    it('returns the params from mixed spaces', async () => {
+      const { service } = getMockedService();
+      jest.spyOn(service, 'getSyntheticsParams').mockRestore();
+
+      serverMock.encryptedSavedObjects = mockEncryptedSO({
+        params: [
+          {
+            attributes: { key: 'username', value: 'elastic' },
+            namespaces: ['default'],
+          },
+          {
+            attributes: { key: 'username-shared', value: 'elastic' },
+            namespaces: ['*'],
+          },
+          {
+            attributes: { key: 'username-test-space', value: 'elastic' },
+            namespaces: ['test'],
+          },
+        ],
+      });
+
+      const params = await service.getSyntheticsParams({ spaceId: 'default' });
+
+      expect(params).toEqual({
+        '*': {
+          'username-shared': 'elastic',
+        },
+        default: {
+          username: 'elastic',
+          'username-shared': 'elastic',
+        },
+        test: {
+          'username-shared': 'elastic',
+          'username-test-space': 'elastic',
+        },
+      });
+    });
   });
 
   describe('pagination', () => {

x-pack/solutions/observability/plugins/synthetics/server/synthetics_service/synthetics_service.ts

@@ -618,7 +618,10 @@ export class SyntheticsService {
     if (paramsBySpace[ALL_SPACES_ID]) {
       Object.keys(paramsBySpace).forEach((space) => {
         if (space !== ALL_SPACES_ID) {
-          paramsBySpace[space] = Object.assign(paramsBySpace[ALL_SPACES_ID], paramsBySpace[space]);
+          paramsBySpace[space] = {
+            ...(paramsBySpace[space] ?? {}),
+            ...(paramsBySpace[ALL_SPACES_ID] ?? {}),
+          };
         }
       });
       if (spaceId) {