[Obs AI Assistant] It should be possible to clear the user-specific s…

…ystem prompt (elastic#202279)

I noticed that I was not able to clear the user-specific system prompt.
I had initially entered "Please speak in Swedish" and saved. Afterwards
I wanted to clear this but the save button is disabled if the text
content is empty.

![image](https://github.com/user-attachments/assets/c3889831-e96a-491f-a8c1-29ae235af2ae)

x-pack/plugins/observability_solution/observability_ai_assistant/server/routes/knowledge_base/route.ts

@@ -133,7 +133,7 @@ const saveKnowledgeBaseUserInstruction = createObservabilityAIAssistantServerRou
   params: t.type({
     body: t.type({
       id: t.string,
-      text: nonEmptyStringRt,
+      text: t.string,
       public: toBooleanRt,
     }),
   }),

x-pack/plugins/observability_solution/observability_ai_assistant/server/service/knowledge_base_service/index.ts

@@ -405,7 +405,7 @@ export class KnowledgeBaseService {
         document: {
           '@timestamp': new Date().toISOString(),
           ...doc,
-          semantic_text: doc.text,
+          ...(doc.text ? { semantic_text: doc.text } : {}),
           user,
           namespace,
         },

x-pack/plugins/observability_solution/observability_ai_assistant_management/public/routes/components/knowledge_base_edit_user_instruction_flyout.tsx

@@ -31,7 +31,6 @@ export function KnowledgeBaseEditUserInstructionFlyout({ onClose }: { onClose: (
   const { mutateAsync: createEntry, isLoading: isSaving } = useCreateKnowledgeBaseUserInstruction();
   const [newEntryText, setNewEntryText] = useState('');
   const [newEntryId, setNewEntryId] = useState<string>();
-  const isSubmitDisabled = newEntryText.trim() === '';
 
   useEffect(() => {
     const userInstruction = userInstructions?.find((entry) => !entry.public);
@@ -118,7 +117,6 @@ export function KnowledgeBaseEditUserInstructionFlyout({ onClose }: { onClose: (
               fill
               isLoading={isSaving}
               onClick={handleSubmit}
-              isDisabled={isSubmitDisabled}
             >
               {i18n.translate(
                 'xpack.observabilityAiAssistantManagement.knowledgeBaseNewManualEntryFlyout.saveButtonLabel',

x-pack/test/observability_ai_assistant_api_integration/tests/knowledge_base/knowledge_base_user_instructions.spec.ts

@@ -331,5 +331,36 @@ export default function ApiTest({ getService }: FtrProviderContext) {
         expect(conversation.messages.length).to.be(5);
       });
     });
+
+    describe('Instructions can be saved and cleared again', () => {
+      async function updateInstruction(text: string) {
+        await observabilityAIAssistantAPIClient
+          .editor({
+            endpoint: 'PUT /internal/observability_ai_assistant/kb/user_instructions',
+            params: {
+              body: {
+                id: 'my-instruction-that-will-be-cleared',
+                text,
+                public: false,
+              },
+            },
+          })
+          .expect(200);
+
+        const res = await observabilityAIAssistantAPIClient
+          .editor({ endpoint: 'GET /internal/observability_ai_assistant/kb/user_instructions' })
+          .expect(200);
+
+        return res.body.userInstructions[0].text;
+      }
+
+      it('can clear the instruction', async () => {
+        const res1 = await updateInstruction('This is a user instruction that will be cleared');
+        expect(res1).to.be('This is a user instruction that will be cleared');
+
+        const res2 = await updateInstruction('');
+        expect(res2).to.be('');
+      });
+    });
   });
 }