fix(event_session): restore event_presentation/presenter_li_qry_str fields

These fields from v_event_session_w_file_count were lost during the v1/v2
-> v3 migration. Added to Event_Session_Base model and to searchable_fields
in the event_session object definition.

Fields are only available via the alt view (v_event_session_w_file_count).
To search: use ?view=alt on the nested search endpoint.
To retrieve: use ?inc_file_count=true on the GET endpoint.

Also:
- Updated ARCH__V3_DEVELOPMENT_STANDARDS.md: expanded Field Evolution
  Checklist with alt-view field rules, Docker restart requirement, and
  documented the ?view= parameter as a live (not proposed) feature.
- Updated TODO__Agents.md: marked migration gap audit as complete.
- Added regression test to test_e2e_v3_search_engine.py.

app/models/event_session_models.py

@@ -139,6 +139,8 @@ class Event_Session_Base(BaseModel):
     created_on: Optional[datetime.datetime] = None
     updated_on: Optional[datetime.datetime] = None
     default_qry_str: Optional[str] # Default query string used for searching and filtering sessions. Updated using SQL triggers and a SQL function
+    event_presentation_li_qry_str: Optional[str] # Concatenated query string of presentation data for this session (from v_event_session_w_file_count)
+    event_presenter_li_qry_str: Optional[str] # Concatenated query string of presenter data for this session (from v_event_session_w_file_count)
 
     # Including convenience data
     # This is only for convenience. Probably going to keep unless it causes a problem.