When WT-14832 is resolved, the `test/model` framework will be able to fuzz test search behaviors in WiredTiger. That ticket's endpoint, however, is to check the result code of the search operation. This is in line with other modeled operations, which also return only result codes, and is useful in its own right. It's not complete, however, since search operations return a value on success.

This ticket is to add support for return values that go beyond error codes to `test/model`, resolving the two FIXME comments tagged with this ticket number. It will require changing the operation result type to an `std::variant` of both the normal `int` return code (which is the current result type, used pervasively throughout the codebase), and a struct containing both the return code and a value. By applying this change consistently and defining appropriate operator overloads, this will allow the tester to record and compare return values, increasing the range of potential issues the fuzzer could help identify.