ES|QL Completion command syntax change (elastic#129189)

Author: afoucret

x-pack/plugin/esql/qa/testFixtures/src/main/resources/completion.csv-spec

@@ -6,7 +6,7 @@ completion using a ROW source operator
 required_capability: completion
 
 ROW prompt="Who is Victor Hugo?"
-| COMPLETION prompt WITH test_completion AS completion_output
+| COMPLETION completion_output = prompt WITH test_completion
 ;
 
 prompt:keyword      | completion_output:keyword
@@ -18,7 +18,7 @@ completion using a ROW source operator and prompt is a multi-valued field
 required_capability: completion
 
 ROW prompt=["Answer the following question:", "Who is Victor Hugo?"]
-| COMPLETION prompt WITH test_completion AS completion_output
+| COMPLETION completion_output = prompt WITH test_completion
 ;
 
 prompt:keyword                                        | completion_output:keyword

x-pack/plugin/esql/src/main/antlr/EsqlBaseLexer.g4

@@ -180,7 +180,6 @@ DECIMAL_LITERAL
 
 
 AND : 'and';
-AS: 'as';
 ASC : 'asc';
 ASSIGN : '=';
 BY : 'by';
@@ -370,7 +369,7 @@ RENAME_NAMED_OR_POSITIONAL_PARAM : NAMED_OR_POSITIONAL_PARAM -> type(NAMED_OR_PO
 RENAME_DOUBLE_PARAMS : DOUBLE_PARAMS -> type(DOUBLE_PARAMS);
 RENAME_NAMED_OR_POSITIONAL_DOUBLE_PARAMS : NAMED_OR_POSITIONAL_DOUBLE_PARAMS -> type(NAMED_OR_POSITIONAL_DOUBLE_PARAMS);
 
-RENAME_AS : AS -> type(AS);
+AS: 'as';
 
 RENAME_ID_PATTERN
     : ID_PATTERN -> type(ID_PATTERN)

x-pack/plugin/esql/src/main/antlr/EsqlBaseLexer.tokens

@@ -382,5 +382,5 @@ rerankCommand
     ;
 
 completionCommand
-    : COMPLETION prompt=primaryExpression WITH inferenceId=identifierOrParameter (AS targetField=qualifiedName)?
+    : COMPLETION (targetField=qualifiedName ASSIGN)? prompt=primaryExpression WITH inferenceId=identifierOrParameter
     ;