[ML] API integration tests for /trained_models/model_downloads endpoint (#187865)

## Summary

Part of #182235

Adds API integration tests for the `GET
/internal/ml/trained_models/model_downloads` endpoint


### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

x-pack/test/api_integration/apis/ml/trained_models/index.ts

@@ -15,5 +15,6 @@ export default function ({ loadTestFile }: FtrProviderContext) {
     loadTestFile(require.resolve('./delete_model'));
     loadTestFile(require.resolve('./put_model'));
     loadTestFile(require.resolve('./start_stop_deployment'));
+    loadTestFile(require.resolve('./model_downloads'));
   });
 }

x-pack/test/api_integration/apis/ml/trained_models/model_downloads.ts

@@ -0,0 +1,136 @@
+/*
+ * 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 { type NodesInfoNodeInfo } from '@elastic/elasticsearch/lib/api/typesWithBodyKey';
+import { FtrProviderContext } from '../../../ftr_provider_context';
+import { USER } from '../../../../functional/services/ml/security_common';
+import { getCommonRequestHeader } from '../../../../functional/services/ml/common_api';
+
+export default ({ getService }: FtrProviderContext) => {
+  const supertest = getService('supertestWithoutAuth');
+  const ml = getService('ml');
+  const deployment = getService('deployment');
+  const esSupertest = getService('esSupertest');
+
+  describe('GET trained_models/model_downloads', function () {
+    before(async () => {
+      await ml.api.initSavedObjects();
+      await ml.testResources.setKibanaTimeZoneToUTC();
+    });
+
+    after(async () => {
+      await ml.api.cleanMlIndices();
+    });
+
+    it('returns the list of models available for download', async () => {
+      const isCloud = await deployment.isCloud();
+
+      const { body: mlNodesResponse } = await esSupertest.get('/_nodes/ml:true/os');
+
+      // Check that all ML nodes are Intel-based.
+      const areMlNodesIntelBased = Object.values(
+        mlNodesResponse.nodes as Record<string, NodesInfoNodeInfo>
+      ).every((node) => node.os?.name === 'Linux' && node.os?.arch === 'amd64');
+
+      const isIntelBased = isCloud || areMlNodesIntelBased;
+
+      const { body, status } = await supertest
+        .get(`/internal/ml/trained_models/model_downloads`)
+        .auth(USER.ML_POWERUSER, ml.securityCommon.getPasswordForUser(USER.ML_POWERUSER))
+        .set(getCommonRequestHeader('1'));
+      ml.api.assertResponseStatusCode(200, status, body);
+
+      expect(body.length).to.eql(5);
+
+      expect(body).to.eql([
+        {
+          modelName: 'elser',
+          hidden: true,
+          version: 1,
+          config: {
+            input: {
+              field_names: ['text_field'],
+            },
+          },
+          description: 'Elastic Learned Sparse EncodeR v1 (Tech Preview)',
+          type: ['elastic', 'pytorch', 'text_expansion'],
+          model_id: '.elser_model_1',
+        },
+        {
+          modelName: 'elser',
+          version: 2,
+          config: {
+            input: {
+              field_names: ['text_field'],
+            },
+          },
+          description: 'Elastic Learned Sparse EncodeR v2',
+          type: ['elastic', 'pytorch', 'text_expansion'],
+          model_id: '.elser_model_2',
+          ...(isIntelBased ? { default: true } : { recommended: true }),
+        },
+        {
+          modelName: 'elser',
+          version: 2,
+          os: 'Linux',
+          arch: 'amd64',
+          config: {
+            input: {
+              field_names: ['text_field'],
+            },
+          },
+          description: 'Elastic Learned Sparse EncodeR v2, optimized for linux-x86_64',
+          type: ['elastic', 'pytorch', 'text_expansion'],
+          model_id: '.elser_model_2_linux-x86_64',
+          ...(isIntelBased ? { recommended: true } : {}),
+        },
+        {
+          modelName: 'e5',
+          version: 1,
+          config: {
+            input: {
+              field_names: ['text_field'],
+            },
+          },
+          description: 'E5 (EmbEddings from bidirEctional Encoder rEpresentations)',
+          license: 'MIT',
+          licenseUrl: 'https://huggingface.co/elastic/multilingual-e5-small',
+          type: ['pytorch', 'text_embedding'],
+          model_id: '.multilingual-e5-small',
+          ...(isIntelBased ? { default: true } : { recommended: true }),
+        },
+        {
+          modelName: 'e5',
+          version: 1,
+          os: 'Linux',
+          arch: 'amd64',
+          config: {
+            input: {
+              field_names: ['text_field'],
+            },
+          },
+          description:
+            'E5 (EmbEddings from bidirEctional Encoder rEpresentations), optimized for linux-x86_64',
+          license: 'MIT',
+          licenseUrl: 'https://huggingface.co/elastic/multilingual-e5-small_linux-x86_64',
+          type: ['pytorch', 'text_embedding'],
+          model_id: '.multilingual-e5-small_linux-x86_64',
+          ...(isIntelBased ? { recommended: true } : {}),
+        },
+      ]);
+    });
+
+    it('returns an error for unauthorized user', async () => {
+      const { body, status } = await supertest
+        .get(`/internal/ml/trained_models/model_downloads`)
+        .auth(USER.ML_UNAUTHORIZED, ml.securityCommon.getPasswordForUser(USER.ML_UNAUTHORIZED))
+        .set(getCommonRequestHeader('1'));
+      ml.api.assertResponseStatusCode(403, status, body);
+    });
+  });
+};