latest_prices: add include_expired passthrough for historical data

ag-monitor now drops expired delivery months from /api/data/latest by
default (current + future only). Expose the opt-in through the MCP so the
advisor can fetch past months when a farmer explicitly asks for history,
while the default stays current + future.

- client.latest(include_expired=False): forwards include_expired=true only
  when set (None is dropped), so the default query string is unchanged.
- latest_prices tool: new include_expired arg with a description steering
  the model to set it only on an explicit historical request.

Tests: client passthrough (default omits the param; opt-in forwards it).
43 passed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ag_bids_mcp/client.py

@@ -56,10 +56,14 @@ def _get(path: str, **params: Any) -> dict | list:
 def latest(commodity: str | None = None, source: str | None = None,
            delivery: str | None = None, kind: str | None = None,
            zip: str | None = None, lat: float | None = None,
-           lng: float | None = None, radius_miles: float | None = None) -> dict:
+           lng: float | None = None, radius_miles: float | None = None,
+           include_expired: bool = False) -> dict:
+    # Only send include_expired when True — _get drops None, so the default
+    # call keeps its existing query string (current + future months only).
     return _get("/api/data/latest",
                 commodity=commodity, source=source, delivery=delivery, kind=kind,
-                zip=zip, lat=lat, lng=lng, radius_miles=radius_miles)
+                zip=zip, lat=lat, lng=lng, radius_miles=radius_miles,
+                include_expired=True if include_expired else None)
 
 
 def history(commodity: str | None = None, source_id: int | None = None,