[Fleet] Fix namespaces property of created agent policies (#189199)

## Summary

I found a small bug while working on
#185040: when agent policies are
created, there should be a root-level `namespaces` property, which is
currently missing.

`GET .fleet-policies/_mapping` contains a `namespaces` property with
`keyword` type that was added in
https://github.com/elastic/elasticsearch.

Note: I was looking into removing the existing `data.namespaces`
property, however I don't see any issues with it. It is coming from
[here](https://github.com/nchaulet/kibana/blob/f77e4d243fca87a87eeae1409f27876cc7ea0836/x-pack/plugins/fleet/server/services/agent_policy.ts#L1140),
i.e. the `data` property is generated from the full agent policy which
already has a `namespaces` property.

### 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/plugins/fleet/common/types/models/agent_policy.ts

@@ -193,6 +193,10 @@ export interface FleetServerPolicy {
    * The coordinator index of the policy
    */
   coordinator_idx: number;
+  /**
+   * The namespaces of the policy
+   */
+  namespaces?: string[];
   /**
    * The opaque payload.
    */

x-pack/plugins/fleet/server/services/agent_policy.test.ts

@@ -1230,6 +1230,7 @@ describe('Agent policy', () => {
       mockedGetFullAgentPolicy.mockResolvedValue({
         id: 'policy123',
         revision: 1,
+        namespaces: ['mySpace'],
         inputs: [
           {
             id: 'input-123',
@@ -1282,10 +1283,16 @@ describe('Agent policy', () => {
             }),
             expect.objectContaining({
               '@timestamp': expect.anything(),
-              data: { id: 'policy123', inputs: [{ id: 'input-123' }], revision: 1 },
+              data: {
+                id: 'policy123',
+                inputs: [{ id: 'input-123' }],
+                revision: 1,
+                namespaces: ['mySpace'],
+              },
               default_fleet_server: false,
               policy_id: 'policy123',
               revision_idx: 1,
+              namespaces: ['mySpace'],
             }),
           ],
           refresh: 'wait_for',

x-pack/plugins/fleet/server/services/agent_policy.ts

@@ -1137,6 +1137,7 @@ class AgentPolicyService {
         '@timestamp': new Date().toISOString(),
         revision_idx: fullPolicy.revision,
         coordinator_idx: 0,
+        namespaces: fullPolicy.namespaces,
         data: fullPolicy as unknown as FleetServerPolicy['data'],
         policy_id: fullPolicy.id,
         default_fleet_server: policy.is_default_fleet_server === true,