[Obs AI Assistant] Include an AdHoc instruction about the slack connector to avoid executing a loop (#199531)

Closes https://github.com/elastic/kibana/issues/185028

## Summary

### Problem
The ticket mentions that there was an issue with displaying results.
However both `display results` and `visualize query` are working as
expected based on my investigation. More details including a video are
attached
[here](https://github.com/elastic/kibana/issues/185028#issuecomment-2464879452).

The function calling loop seems to occur when the AI Assistant is trying
to send the output to Slack via the Kibana Slack connector. In order to
do this, the LLM invokes the function `execute_connector`. For the Slack
connector, `id` and `params` properties are required. However, the LLM
only populated `id` and not `params` which causes an error when
validated against the schema for the Slack connector.

- Sometimes, it's able to retry a few times and successfully send the
output to Slack.
- Sometimes, it goes into a loop trying to find `params` and failing
repeatedly.

Attaching another similar issue for more context -
https://github.com/elastic/kibana/issues/195564

_With the solution below, my intention is to send the output to Slack in
one go, without retrying the function `execute_connector`._

### Solution
Based on the solutions I experimented with, seems like we need to force
the LLM to understand what's needed for the Slack connector. I tried a
few options here and the combination of updates that worked are as
follows:
- Appending an AdHoc instruction about the Slack connector properties.
(This gives the LLM some additional information about the required
properties)
- Updating the `properties` attached to the connector (when passing the
connector list to the LLM), to reflect both `id` and `params` with
`message`.

With the above change, the AI Assistant has managed to consistently send
the output to Slack _**without any retries**_ because of missing
`params`.

### Screenshots:
(all alert triggers successfully sent the output to Slack without having
to retry the function)

<img width="1452" alt="success-attempts-to-slack-connector"
src="https://github.com/user-attachments/assets/715a5957-2c04-4a55-868f-34abe564f6d4">

x-pack/plugins/observability_solution/observability_ai_assistant/server/functions/execute_connector.ts

@@ -8,13 +8,15 @@
 import { FunctionRegistrationParameters } from '.';
 import { FunctionVisibility } from '../../common';
 
+export const EXECUTE_CONNECTOR_FUNCTION_NAME = 'execute_connector';
+
 export function registerExecuteConnectorFunction({
   functions,
   resources,
 }: FunctionRegistrationParameters) {
   functions.registerFunction(
     {
-      name: 'execute_connector',
+      name: EXECUTE_CONNECTOR_FUNCTION_NAME,
       description: 'Use this function when user explicitly asks to call a kibana connector.',
       visibility: FunctionVisibility.AssistantOnly,
       parameters: {

x-pack/plugins/observability_solution/observability_ai_assistant_app/server/rule_connector/index.ts

@@ -23,7 +23,6 @@ import {
   JiraParamsSchema,
   PagerdutyParamsSchema,
   SlackApiParamsSchema,
-  SlackParamsSchema,
   WebhookParamsSchema,
 } from '@kbn/stack-connectors-plugin/server';
 import { ObservabilityAIAssistantRouteHandlerResources } from '@kbn/observability-ai-assistant-plugin/server/routes/types';
@@ -37,14 +36,26 @@ import { CompatibleJSONSchema } from '@kbn/observability-ai-assistant-plugin/com
 import { AlertDetailsContextualInsightsService } from '@kbn/observability-plugin/server/services';
 import { getSystemMessageFromInstructions } from '@kbn/observability-ai-assistant-plugin/server/service/util/get_system_message_from_instructions';
 import { AdHocInstruction } from '@kbn/observability-ai-assistant-plugin/common/types';
+import { EXECUTE_CONNECTOR_FUNCTION_NAME } from '@kbn/observability-ai-assistant-plugin/server/functions/execute_connector';
 import { convertSchemaToOpenApi } from './convert_schema_to_open_api';
 import { OBSERVABILITY_AI_ASSISTANT_CONNECTOR_ID } from '../../common/rule_connector';
 
 const CONNECTOR_PRIVILEGES = ['api:observabilityAIAssistant', 'app:observabilityAIAssistant'];
 
 const connectorParamsSchemas: Record<string, CompatibleJSONSchema> = {
+  '.slack': {
+    type: 'object',
+    properties: {
+      id: { type: 'string' },
+      params: {
+        type: 'object',
+        properties: {
+          message: { type: 'string' },
+        },
+      },
+    },
+  },
   '.slack_api': convertSchemaToOpenApi(SlackApiParamsSchema),
-  '.slack': convertSchemaToOpenApi(SlackParamsSchema),
   '.email': convertSchemaToOpenApi(EmailParamsSchema),
   '.webhook': convertSchemaToOpenApi(WebhookParamsSchema),
   '.jira': convertSchemaToOpenApi(JiraParamsSchema),
@@ -189,6 +200,24 @@ If available, include the link of the conversation at the end of your answer.`
     ),
   };
 
+  const hasSlackConnector = !!connectorsList.filter(
+    (connector) => connector.actionTypeId === '.slack'
+  ).length;
+
+  if (hasSlackConnector && functionClient.hasFunction(EXECUTE_CONNECTOR_FUNCTION_NAME)) {
+    const slackConnectorInstruction: AdHocInstruction = {
+      instruction_type: 'application_instruction',
+      text: dedent(
+        `The execute_connector function can be used to invoke Kibana connectors.
+        To send to the Slack connector, you need the following arguments:
+          - the "id" of the connector
+          - the "params" parameter that you will fill with the message
+        Please include both "id" and "params.message" in the function arguments when executing the Slack connector..`
+      ),
+    };
+    functionClient.registerAdhocInstruction(slackConnectorInstruction);
+  }
+
   const alertsContext = await getAlertsContext(
     execOptions.params.rule,
     execOptions.params.alerts,