Fix iteration tests: add handlers, responses, and requests

[p123-stack]

Add handlers, request, and response classes for the testkit backend so iteration tests pass and match the expected driver behavior.

Handlers and classes:

• ResultList – Iterates over results and returns RecordListResponse; uses a while loop instead of foreach so it continues from the current position when next() was called first.
• ResultSingle – Returns exactly one record or a DriverError when the count is not 1; registered in RequestFactory.
• ResultSingleOptional – Returns one record or null when there are 0 or more than 1; uses a while loop to avoid rewind on already-consumed results.
• ResultPeek – Returns the next record without advancing the iterator.
• ResultNext – Advances the iterator after returning the current record so next() and list() share the same position.

Expected driver behavior:

• result.next() advances the iterator; result.list() continues from the current position and does not rewind.
• result.single() consumes the result and enforces exactly one record.
• result.single_optional() consumes the result and returns one record or null.
• result.peek() returns the next record without consuming it.