[ES|QL] Implement RERANK Ast Parser + basic template (elastic#232885)

## Summary

Part of: elastic#217285

Grammar:

` rerankCommand: RERANK (targetField=qualifiedName ASSIGN)?
queryText=constant ON rerankFields commandNamedParameters
`

Grammar -> commandParameters expanded:

`commandNamedParameters: (WITH mapExpression)?`
`mapExpression:
LEFT_BRACES (entryExpression (COMMA entryExpression)*)? RIGHT_BRACES`

Syntax: 

`RERANK [column =] query ON field [, field, ...] [WITH { "inference_id"
: "my_inference_endpoint" }]`

Author: bartoval

src/platform/packages/shared/kbn-esql-ast/src/commands_registry/commands/rerank/autocomplete.ts

@@ -0,0 +1,36 @@
+/*
+ * 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", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+import type { ESQLCommand } from '../../../types';
+import { pipeCompleteItem } from '../../complete_items';
+import type { ICommandCallbacks } from '../../types';
+import { type ISuggestionItem, type ICommandContext } from '../../types';
+
+export async function autocomplete(
+  query: string,
+  command: ESQLCommand,
+  callbacks?: ICommandCallbacks,
+  context?: ICommandContext,
+  cursorPosition?: number
+): Promise<ISuggestionItem[]> {
+  if (!callbacks?.getByType) {
+    return [];
+  }
+
+  const suggestions: ISuggestionItem[] = [];
+
+  // TODO: Add more specific autocomplete logic for RERANK command
+  const fieldSuggestions = await callbacks.getByType('any');
+  suggestions.push(...fieldSuggestions);
+
+  // Add the pipe completion item when appropriate
+  suggestions.push(pipeCompleteItem);
+
+  return suggestions;
+}

src/platform/packages/shared/kbn-esql-ast/src/commands_registry/commands/rerank/index.ts

@@ -0,0 +1,38 @@
+/*
+ * 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", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+import { i18n } from '@kbn/i18n';
+import type { ICommandMethods } from '../../registry';
+import { autocomplete } from './autocomplete';
+import { validate } from './validate';
+import type { ICommandContext } from '../../types';
+
+const rerankCommandMethods: ICommandMethods<ICommandContext> = {
+  autocomplete,
+  validate,
+};
+
+export const rerankCommand = {
+  name: 'rerank',
+  methods: rerankCommandMethods,
+  hidden: true,
+  metadata: {
+    description: i18n.translate('kbn-esql-ast.esql.definitions.rerankDoc', {
+      defaultMessage:
+        'Uses an inference model to compute new relevance scores for documents, directly within your ES|QL queries.',
+    }),
+    declaration:
+      'RERANK [target_field =] query_text ON field1 [, field2, ...] WITH { "inference_id": "model_id" }',
+    examples: [
+      'FROM movies | RERANK "star wars" ON title WITH { "inference_id": "reranker" }',
+      'FROM books | RERANK rerank_score = "hobbit" ON title, description WITH { "inference_id": "my_reranker" }',
+    ],
+    preview: true,
+  },
+};

src/platform/packages/shared/kbn-esql-ast/src/commands_registry/commands/rerank/validate.ts

@@ -0,0 +1,54 @@
+/*
+ * 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", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+import { i18n } from '@kbn/i18n';
+import type { ESQLCommand, ESQLMessage, ESQLAst, ESQLAstRerankCommand } from '../../../types';
+import type { ICommandContext, ICommandCallbacks } from '../../types';
+import { validateCommandArguments } from '../../../definitions/utils/validation';
+
+export const validate = (
+  command: ESQLCommand,
+  ast: ESQLAst,
+  context?: ICommandContext,
+  callbacks?: ICommandCallbacks
+): ESQLMessage[] => {
+  const messages: ESQLMessage[] = [];
+
+  // Run standard argument validation
+  messages.push(...validateCommandArguments(command, ast, context, callbacks));
+
+  // Cast to RERANK command for type-specific validation
+  const rerankCommand = command as ESQLAstRerankCommand;
+
+  // Validate that query text is provided
+  if (!rerankCommand.query) {
+    messages.push({
+      location: command.location,
+      text: i18n.translate('kbn-esql-ast.esql.validation.rerankMissingQuery', {
+        defaultMessage: '[RERANK] Query text is required.',
+      }),
+      type: 'error',
+      code: 'rerankMissingQuery',
+    });
+  }
+
+  // Validate that at least one field is specified in ON clause
+  if (!rerankCommand.fields || rerankCommand.fields.length === 0) {
+    messages.push({
+      location: command.location,
+      text: i18n.translate('kbn-esql-ast.esql.validation.rerankMissingFields', {
+        defaultMessage: '[RERANK] At least one field must be specified in the ON clause.',
+      }),
+      type: 'error',
+      code: 'rerankMissingFields',
+    });
+  }
+
+  return messages;
+};

src/platform/packages/shared/kbn-esql-ast/src/commands_registry/index.ts

@@ -31,6 +31,7 @@ import { showCommand } from './commands/show';
 import { timeseriesCommand } from './commands/timeseries';
 import { whereCommand } from './commands/where';
 import { fuseCommand } from './commands/fuse';
+import { rerankCommand } from './commands/rerank';
 import { mergeCommandWithGeneratedCommandData } from './elastisearch_command_data_loader';
 
 const esqlCommandRegistry = new CommandRegistry();
@@ -59,6 +60,7 @@ const baseCommands = [
   timeseriesCommand,
   whereCommand,
   fuseCommand,
+  rerankCommand,
 ];
 
 baseCommands.forEach((command) => {