Description

When running an aggregation containing a $scoreFusion stage, supplying combination.expression while combination.method is not "expression" is silently accepted: the server applies the default avg method and drops the expression value without returning any error.

Verified on Server 8.3.0.

Expected behavior

The server should return a clear error rejecting the request when combination.expression is supplied without combination.method == "expression". This matches the rejection behavior for other invalid $scoreFusion configurations (e.g., combination.method: "sum" is rejected with BadValue).

Reproduction (mongosh)

db.createCollection("coll");
db.coll.insertMany([{ x: 1 }, { x: 2 }, { x: 3 }]);

db.coll.aggregate([
  {
    $scoreFusion: {
      input: {
        pipelines: {
          p1: [{ $score: { score: "$x", normalization: "none" } }]
        },
        normalization: "none"
      },
      combination: {
        // method: omitted; server defaults to "avg"
        expression: { $sum: ["$$p1"] }
      }
    }
  }
]);

The pipeline runs successfully and returns results; the expression is silently dropped. Setting method: "avg" explicitly produces the same silent-drop behavior.