gap Field handling in GapAgent

Is your feature request related to a problem?

yes. Currently the planner uses the Field definitions and type hints to suggest values based on user query. As it stands, we use `str` for gap agent for "gap" input which may have undesired consequences.

Describe the solution you'd like

replace:
 : str = Field(..., description="Type of gap to investigate.")
with
 : GapTypes = Field(..., description="Type of gap to investigate.")

where GapTypes is a controlled Enum list, with expected gaps as input:

e.g. list


Knowledge Gap
Theoretical Gap
Methodological Gap
Population Gap
Geographical Gap

Describe alternatives you've considered

if it is intended to be free form, the description needs to be updated so the planner exactly knows what to fill in based on the request. Also, We may need to consider Default string - i.e. evidence gap

Additional context

user input (which may contain an explicit request for a gap analysis) -> planner -> a json with IO types filled that is used to initialize the agent.

Start by locating GapAgent's gap field and the planner-to-agent JSON initialization path described in the issue. Check how the current str annotation is used and whether a controlled GapTypes value set fits the existing inputs. Done means the accepted gap types and any default or free-form behavior are explicitly defined and the planner passes them correctly.