[Obs AI Assistant] Add knowledge base migration test to the serverless test suite (#205631)

Closes https://github.com/elastic/kibana/issues/205537

## Summary

The knowledge base migration test suite is missing in serverless. This
PR adds it to the serverless test suite.
- This has a dependancy to https://github.com/elastic/kibana/pull/205194
since we are removing all serverless tests and adding them to DA tests.
- If the DA tests PR gets merged first, I'll refactor this PR to add it
there.

### Checklist

- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

x-pack/test_serverless/api_integration/test_suites/observability/ai_assistant/tests/knowledge_base/knowledge_base_migration.spec.ts

@@ -0,0 +1,162 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+import expect from '@kbn/expect';
+import {
+  deleteInferenceEndpoint,
+  createKnowledgeBaseModel,
+  TINY_ELSER,
+  deleteKnowledgeBaseModel,
+  clearKnowledgeBase,
+} from '@kbn/test-suites-xpack/observability_ai_assistant_api_integration/tests/knowledge_base/helpers';
+import { AI_ASSISTANT_KB_INFERENCE_ID } from '@kbn/observability-ai-assistant-plugin/server/service/inference_endpoint';
+import { SearchResponse } from '@elastic/elasticsearch/lib/api/typesWithBodyKey';
+import { KnowledgeBaseEntry } from '@kbn/observability-ai-assistant-plugin/common';
+import { orderBy } from 'lodash';
+import { FtrProviderContext } from '../../common/ftr_provider_context';
+
+export default function ApiTest({ getService }: FtrProviderContext) {
+  const ml = getService('ml');
+  const es = getService('es');
+  const esArchiver = getService('esArchiver');
+  const observabilityAIAssistantAPIClient = getService('observabilityAIAssistantAPIClient');
+
+  const archive =
+    'x-pack/test/functional/es_archives/observability/ai_assistant/knowledge_base_8_15';
+
+  async function getKnowledgeBaseEntries() {
+    const res = (await es.search({
+      index: '.kibana-observability-ai-assistant-kb*',
+      body: {
+        query: {
+          match_all: {},
+        },
+      },
+    })) as SearchResponse<
+      KnowledgeBaseEntry & {
+        semantic_text: {
+          text: string;
+          inference: { inference_id: string; chunks: Array<{ text: string; embeddings: any }> };
+        };
+      }
+    >;
+
+    return res.hits.hits;
+  }
+
+  describe('When there are knowledge base entries (from 8.15 or earlier) that does not contain semantic_text embeddings', function () {
+    this.tags(['skipMKI']);
+
+    before(async () => {
+      await clearKnowledgeBase(es);
+      await esArchiver.load(archive);
+      await createKnowledgeBaseModel(ml);
+      await observabilityAIAssistantAPIClient
+        .slsAdmin({
+          endpoint: 'POST /internal/observability_ai_assistant/kb/setup',
+          params: {
+            query: {
+              model_id: TINY_ELSER.id,
+            },
+          },
+        })
+        .expect(200);
+    });
+
+    after(async () => {
+      await clearKnowledgeBase(es);
+      await esArchiver.unload(archive);
+      await deleteKnowledgeBaseModel(ml);
+      await deleteInferenceEndpoint({ es });
+    });
+
+    describe('before migrating', () => {
+      it('the docs do not have semantic_text embeddings', async () => {
+        const hits = await getKnowledgeBaseEntries();
+        const hasSemanticTextEmbeddings = hits.some((hit) => hit._source?.semantic_text);
+        expect(hasSemanticTextEmbeddings).to.be(false);
+      });
+    });
+
+    describe('after migrating', () => {
+      before(async () => {
+        await observabilityAIAssistantAPIClient
+          .slsEditor({
+            endpoint: 'POST /internal/observability_ai_assistant/kb/semantic_text_migration',
+          })
+          .expect(200);
+      });
+
+      it('the docs have semantic_text embeddings', async () => {
+        const hits = await getKnowledgeBaseEntries();
+        const hasSemanticTextEmbeddings = hits.every((hit) => hit._source?.semantic_text);
+        expect(hasSemanticTextEmbeddings).to.be(true);
+
+        expect(
+          orderBy(hits, '_source.title').map(({ _source }) => {
+            const { text, inference } = _source?.semantic_text!;
+
+            return {
+              text,
+              inferenceId: inference.inference_id,
+              chunkCount: inference.chunks.length,
+            };
+          })
+        ).to.eql([
+          {
+            text: 'To infinity and beyond!',
+            inferenceId: AI_ASSISTANT_KB_INFERENCE_ID,
+            chunkCount: 1,
+          },
+          {
+            text: "The user's favourite color is blue.",
+            inferenceId: AI_ASSISTANT_KB_INFERENCE_ID,
+            chunkCount: 1,
+          },
+        ]);
+      });
+
+      it('returns entries correctly via API', async () => {
+        await observabilityAIAssistantAPIClient
+          .slsEditor({
+            endpoint: 'POST /internal/observability_ai_assistant/kb/semantic_text_migration',
+          })
+          .expect(200);
+
+        const res = await observabilityAIAssistantAPIClient
+          .slsEditor({
+            endpoint: 'GET /internal/observability_ai_assistant/kb/entries',
+            params: {
+              query: {
+                query: '',
+                sortBy: 'title',
+                sortDirection: 'asc',
+              },
+            },
+          })
+          .expect(200);
+
+        expect(
+          res.body.entries.map(({ title, text, role, type }) => ({ title, text, role, type }))
+        ).to.eql([
+          {
+            role: 'user_entry',
+            title: 'Toy Story quote',
+            type: 'contextual',
+            text: 'To infinity and beyond!',
+          },
+          {
+            role: 'assistant_summarization',
+            title: "User's favourite color",
+            type: 'contextual',
+            text: "The user's favourite color is blue.",
+          },
+        ]);
+      });
+    });
+  });
+}