[Obs AI Assistant] Don't fail calls where messages have data (#191952)

In #191607, we fixed a bug around
`strictKeysRt` not strictly checking object types in arrays. That
exposed a bug in our `messageRt` type where we define `data`
incorrectly, which means that any conversation that has data (as the
result of some function calls) will fail with a 400. This PR corrects
that mistake.

x-pack/plugins/observability_solution/observability_ai_assistant/server/routes/runtime_types.ts

@@ -17,8 +17,6 @@ import {
   type StarterPrompt,
 } from '../../common/types';
 
-const serializeableRt = t.any;
-
 export const messageRt: t.Type<Message> = t.type({
   '@timestamp': t.string,
   message: t.intersection([
@@ -35,6 +33,7 @@ export const messageRt: t.Type<Message> = t.type({
       content: t.string,
       name: t.string,
       event: t.string,
+      data: t.string,
       function_call: t.intersection([
         t.type({
           name: t.string,
@@ -45,8 +44,7 @@ export const messageRt: t.Type<Message> = t.type({
           ]),
         }),
         t.partial({
-          arguments: serializeableRt,
-          data: serializeableRt,
+          arguments: t.string,
         }),
       ]),
     }),

x-pack/test/observability_ai_assistant_api_integration/tests/complete/complete.spec.ts

@@ -54,6 +54,8 @@ export default function ApiTest({ getService }: FtrProviderContext) {
       message: {
         role: MessageRole.User,
         content: 'Good morning, bot!',
+        // make sure it doesn't 400 on `data` being set
+        data: '{}',
       },
     },
   ];