ES|QL: FUSE syntax fix when using options before SCORE/GROUP/KEY BY

[ioanatia]

There's a bug right now with FUSE, where WITH cannot be used before the other configurations (SCORE BY/GROUP BY/KEY BY):

FROM books METADATA _score, _id, _index
| FORK (WHERE title:"war and peace" | SORT _score DESC| LIMIT 100)
      (WHERE description:"war and peace" | SORT _score DESC| LIMIT 100)
| FUSE WITH {"rank_constant": 45} score by _score

returns:

{
  "error": {
    "root_cause": [
      {
        "type": "parsing_exception",
        "reason": "line 5:36: mismatched input 'score' expecting {<EOF>, '|', 'with', 'group', 'score', 'key'}"
      }
    ],
    "type": "parsing_exception",
    "reason": "line 5:36: mismatched input 'score' expecting {<EOF>, '|', 'with', 'group', 'score', 'key'}",
    "caused_by": {
      "type": "input_mismatch_exception",
      "reason": null
    }
  },
  "status": 400
}

This should not happen, looking at the parser, WITH/KEY BY/SCORE BY/GROUP BY don't need to be provided in any particular order:

elasticsearch/x-pack/plugin/esql/src/main/antlr/EsqlBaseParser.g4

Lines 344 to 353 in 3508148

	fuseCommand
	: FUSE (fuseType=identifier)? (fuseConfiguration)*
	;
	fuseConfiguration
	: SCORE BY score=qualifiedName
	| KEY BY key=fuseKeyByFields
	| GROUP BY group=qualifiedName
	| WITH options=mapExpression
	;

The bug most likely has to do with mixing ANTLR push modes.
After spending some considerable time trying to fix it, I figured we can just follow the same pattern as the other commands that support options (using WITH), that just use EXPRESSION_MODE.
So I removed the custom Fuse.g4 lexer, and just used the Expression lexer like the other commands do 🙃 .
This not just simplifies how we define the syntax for FUSE, but it also fixes the bug we have.

[elasticsearchmachine]

Hi @ioanatia, I've created a changelog YAML for you.