Address review feedback

drop jsontext Pointer.Tokens model, add jsontext tests, fix strings
Fields tests

Co-authored-by: owen-mc <62447351+owen-mc@users.noreply.github.com>

Author: Copilot

go/ql/lib/ext/encoding.json.jsontext.model.yml

@@ -24,7 +24,6 @@ extensions:
       - ["encoding/json/jsontext", "Pointer", True, "AppendToken", "", "", "Argument[0]", "ReturnValue", "taint", "manual"]
       - ["encoding/json/jsontext", "Pointer", True, "LastToken", "", "", "Argument[receiver]", "ReturnValue", "taint", "manual"]
       - ["encoding/json/jsontext", "Pointer", True, "Parent", "", "", "Argument[receiver]", "ReturnValue", "taint", "manual"]
-      - ["encoding/json/jsontext", "Pointer", True, "Tokens", "", "", "Argument[receiver]", "ReturnValue", "taint", "manual"]
       - ["encoding/json/jsontext", "Token", True, "Clone", "", "", "Argument[receiver]", "ReturnValue", "taint", "manual"]
       - ["encoding/json/jsontext", "Token", True, "Float", "", "", "Argument[receiver]", "ReturnValue[0]", "taint", "manual"]
       - ["encoding/json/jsontext", "Token", True, "Float32", "", "", "Argument[receiver]", "ReturnValue[0]", "taint", "manual"]