This change updates Express executor output so keyPattern is emitted as a BSON/JSON object instead of a preformatted string, making it easier to parse and more consistent with regular IXSCAN output.
It also updates related explain/profile behavior and tests to reflect the new object-based keyPattern representation.

Description of Linked Ticket

The Compass team is working on COMPASS-7794 and ran into some challenges processing the explain output of express plans. In particular, the keyPattern field has a different format for express plans that is proving difficult to parse. For express plans, we see the following output:

                {
                        "stage" : "EXPRESS_IXSCAN",
                        "keyPattern" : "{ _id: 1 }",
                        ...
                }, 

and

                {
                        "stage" : "EXPRESS_IXSCAN",
                        "keyPattern" : "{ a: 1 }",
                        ...
                }, 

whereas regular index scans result in the following:

                                "stage" : "IXSCAN",
                                "keyPattern" : {
                                        "a" : 1
                                }, 

The ask is to make the keyPattern field more easily parseable, either by wrapping the field names in quotes:

"keyPattern": "{ \"a\": 1 }", 

or by serializing the pattern as an object as we do in non-express explain output.