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>

tests/test_client.py

@@ -71,6 +71,30 @@ def test_get_drops_none_params(monkeypatch):
     assert captured["params"] == {"commodity": "corn"}
 
 
+def test_latest_include_expired_passthrough(monkeypatch):
+    client = _reload_client(monkeypatch)
+    captured = {}
+
+    class FakeResp:
+        status_code = 200
+        text = ""
+        def json(self): return {"count": 0, "rows": []}
+
+    def fake_get(url, params=None, timeout=None, headers=None):
+        captured["params"] = dict(params or {})
+        return FakeResp()
+
+    monkeypatch.setattr(client.httpx, "get", fake_get)
+
+    # Default: not sent, so the API applies its current+future filter.
+    client.latest(commodity="corn")
+    assert "include_expired" not in captured["params"]
+
+    # Opt in: forwarded so the API returns expired months too.
+    client.latest(commodity="corn", include_expired=True)
+    assert captured["params"] == {"commodity": "corn", "include_expired": True}
+
+
 def test_futures_endpoint_params(monkeypatch):
     client = _reload_client(monkeypatch)
     captured = {}