[Fleet] fix response schema for cancel upgrade (#205493)

Noticed when cancelling an upgrade action that the API gave an error
response saying that `agents` field is expected to be an array, but got
undefined.
Fixing the schema so that `agents` is optional.

Verified by scheduling an upgrade in the future and cancelling from UI.

<img width="2535" alt="image"
src="https://github.com/user-attachments/assets/556ac459-9f46-4a69-aa3a-39e20d9cf55b"
/>

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

oas_docs/bundle.json

@@ -16520,8 +16520,7 @@
                         "type",
                         "data",
                         "created_at",
-                        "ack_data",
-                        "agents"
+                        "ack_data"
                       ],
                       "type": "object"
                     }
@@ -18740,8 +18739,7 @@
                         "type",
                         "data",
                         "created_at",
-                        "ack_data",
-                        "agents"
+                        "ack_data"
                       ],
                       "type": "object"
                     }

oas_docs/bundle.serverless.json

@@ -16520,8 +16520,7 @@
                         "type",
                         "data",
                         "created_at",
-                        "ack_data",
-                        "agents"
+                        "ack_data"
                       ],
                       "type": "object"
                     }
@@ -18740,8 +18739,7 @@
                         "type",
                         "data",
                         "created_at",
-                        "ack_data",
-                        "agents"
+                        "ack_data"
                       ],
                       "type": "object"
                     }

oas_docs/output/kibana.serverless.yaml

@@ -17250,7 +17250,6 @@ paths:
                       - data
                       - created_at
                       - ack_data
-                      - agents
                 required:
                   - item
         '400':
@@ -17779,7 +17778,6 @@ paths:
                       - data
                       - created_at
                       - ack_data
-                      - agents
                 required:
                   - item
         '400':

oas_docs/output/kibana.yaml

@@ -19374,7 +19374,6 @@ paths:
                       - data
                       - created_at
                       - ack_data
-                      - agents
                 required:
                   - item
         '400':
@@ -19896,7 +19895,6 @@ paths:
                       - data
                       - created_at
                       - ack_data
-                      - agents
                 required:
                   - item
         '400':

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

@@ -253,7 +253,7 @@ export const PostNewAgentActionResponseSchema = schema.object({
     sent_at: schema.maybe(schema.string()),
     created_at: schema.string(),
     ack_data: schema.maybe(schema.any()),
-    agents: schema.arrayOf(schema.string()),
+    agents: schema.maybe(schema.arrayOf(schema.string())),
     namespaces: schema.maybe(schema.arrayOf(schema.string())),
     expiration: schema.maybe(schema.string()),
     start_time: schema.maybe(schema.string()),