[Fleet] fix reassign agent with spaces (#214748)

## Summary

Closes https://github.com/elastic/kibana/issues/214341

Updating agent spaces when reassigning to agent policy.

To verify:
- Enable space awareness
- Create an agent policy and enroll an agent to it
- Create another agent policy and assign 2 spaces to it
- Reassign agent to the second policy
- Verify that the agent is visible in both spaces
- Same behaviour with bulk reassign action

<img width="1789" alt="image"
src="https://github.com/user-attachments/assets/3c7e6f07-c8d1-4a26-9e05-483caea6da1e"
/>
<img width="1789" alt="image"
src="https://github.com/user-attachments/assets/b84d2f69-8ba7-4a38-9a80-9b9e8ab9b37a"
/>
<img width="1782" alt="image"
src="https://github.com/user-attachments/assets/1272c35b-25cc-4a55-af6d-61b442ff3e07"
/>
<img width="1489" alt="image"
src="https://github.com/user-attachments/assets/df23f420-63f5-4d4b-bdb2-721a381adb4e"
/>


### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

x-pack/platform/plugins/shared/fleet/server/services/agents/action.mock.ts

@@ -86,11 +86,11 @@ export function createClientMock() {
   };
   const regularAgentPolicySO = {
     id: 'regular-agent-policy',
-    attributes: { is_managed: false },
+    attributes: { is_managed: false, space_ids: ['space1'] },
   } as SavedObject<AgentPolicy>;
   const regularAgentPolicySO2 = {
     id: 'regular-agent-policy-2',
-    attributes: { is_managed: false },
+    attributes: { is_managed: false, space_ids: ['space1', 'default'] },
   } as SavedObject<AgentPolicy>;
   const hostedAgentPolicySO = {
     id: 'hosted-agent-policy',

x-pack/platform/plugins/shared/fleet/server/services/agents/reassign.test.ts

@@ -71,6 +71,20 @@ describe('reassignAgent', () => {
       // does not call ES update
       expect(esClient.update).toBeCalledTimes(0);
     });
+
+    it('update namespaces with reassign', async () => {
+      const { soClient, esClient, agentInRegularDoc, regularAgentPolicySO } = mocks;
+
+      await reassignAgent(soClient, esClient, agentInRegularDoc._id, regularAgentPolicySO.id);
+
+      // calls ES update with correct values
+      expect(esClient.update).toBeCalledTimes(1);
+      const calledWith = esClient.update.mock.calls[0];
+      expect(calledWith[0]?.id).toBe(agentInRegularDoc._id);
+      expect((calledWith[0] as estypes.UpdateRequest)?.doc).toHaveProperty('namespaces', [
+        'space1',
+      ]);
+    });
   });
 
   describe('reassignAgents (plural)', () => {
@@ -104,6 +118,10 @@ describe('reassignAgent', () => {
       expect((calledWith as estypes.BulkRequest).operations?.length).toBe(2);
       // @ts-expect-error
       expect(calledWith.operations[0].update._id).toEqual(agentInRegularDoc._id);
+      expect((calledWith.operations?.[1] as any)?.doc).toHaveProperty('namespaces', [
+        'space1',
+        'default',
+      ]);
 
       // hosted policy is updated in action results with error
       const calledWithActionResults = esClient.bulk.mock.calls[1][0] as estypes.BulkRequest;

x-pack/platform/plugins/shared/fleet/server/services/agents/reassign.ts

@@ -73,9 +73,12 @@ export async function reassignAgent(
     );
   }
 
+  const newAgentPolicy = await agentPolicyService.get(soClient, newAgentPolicyId);
+
   await updateAgent(esClient, agentId, {
     policy_id: newAgentPolicyId,
     policy_revision: null,
+    ...(newAgentPolicy?.space_ids ? { namespaces: newAgentPolicy.space_ids } : {}),
   });
 
   const currentSpaceId = getCurrentNamespace(soClient);

x-pack/platform/plugins/shared/fleet/server/services/agents/reassign_action_runner.ts

@@ -13,6 +13,8 @@ import { AgentReassignmentError, HostedAgentPolicyRestrictionRelatedError } from
 
 import { appContextService } from '../app_context';
 
+import { agentPolicyService } from '../agent_policy';
+
 import { ActionRunner } from './action_runner';
 
 import { bulkUpdateAgents } from './crud';
@@ -74,6 +76,8 @@ export async function reassignBatch(
     throw new AgentReassignmentError('No agents to reassign, already assigned or hosted agents');
   }
 
+  const newAgentPolicy = await agentPolicyService.get(soClient, options.newAgentPolicyId);
+
   await bulkUpdateAgents(
     esClient,
     agentsToUpdate.map((agent) => ({
@@ -81,6 +85,7 @@ export async function reassignBatch(
       data: {
         policy_id: options.newAgentPolicyId,
         policy_revision: null,
+        ...(newAgentPolicy?.space_ids ? { namespaces: newAgentPolicy.space_ids } : {}),
       },
     })),
     errors

x-pack/platform/plugins/shared/fleet/server/types/so_attributes.ts

@@ -92,6 +92,7 @@ export interface AgentSOAttributes {
   tags?: string[];
   components?: FleetServerAgentComponent[];
   packages?: string[];
+  namespaces?: string[];
 }
 
 export interface FleetProxySOAttributes {