Resolve "Add fee_schedule/schedules support to Spot User.get_trade_volume" (#454)

## Summary

- Adds a `fee_schedule` parameter to `User.get_trade_volume`, which
requests the `schedules` field from Kraken's `GetTradeVolume` endpoint
(added 2026-06-16). This is the replacement for the `fees`/`fees_maker`
fields on `AssetPairs`, deprecated on 2026-09-08.
- Kraken validates `fee_schedule` strictly against the JSON boolean
literals `"true"`/`"false"` and rejects Python's default `str(bool)`
capitalization (`"True"`/`"False"`) with `EGeneral:Invalid arguments`.
Verified live against the real API; the value is now serialized
correctly.

Closes #452

Author: btschwertfeger

src/kraken/spot/user.py

@@ -926,6 +926,7 @@ def get_trade_volume(
         pair: str | list[str] | None = None,
         *,
         fee_info: bool = True,
+        fee_schedule: bool | None = None,
         extra_params: dict | None = None,
     ) -> dict:
         """
@@ -940,6 +941,12 @@ def get_trade_volume(
         :type pair: str | list[str], optional
         :param fee_info: Include fee information or not (default: ``True``)
         :type fee_info: bool, optional
+        :param fee_schedule: Include the full fee schedule per trading pair in
+            the response's ``schedules`` field (default: ``None``, i.e. not
+            requested). This is the replacement for the deprecated
+            ``fees``/``fees_maker`` fields on
+            :func:`kraken.spot.Market.get_asset_pairs`.
+        :type fee_schedule: bool, optional
 
         .. code-block:: python
             :linenos:
@@ -979,11 +986,40 @@ def get_trade_volume(
                     }
                 }
             }
+            >>> u.get_trade_volume(pair="DOTUSD", fee_schedule=True)
+            {
+                'currency': 'ZUSD',
+                'volume': '212243.1210',
+                'fees': {...},
+                'fees_maker': {...},
+                'schedules': [
+                    {
+                        'pair': 'DOTUSD',
+                        'class': 'forex',
+                        'tiers': [
+                            {'maker_fee': '0.4000', 'taker_fee': '0.8000', 'active': True},
+                            {
+                                'maker_fee': '0.3000',
+                                'taker_fee': '0.6000',
+                                'min_futures_volume': '5000000.0000',
+                                'min_spot_volume': '2500.0000'
+                            },
+                            ...
+                        ]
+                    }
+                ]
+            }
 
         """
         params: dict = {"fee-info": fee_info}
         if defined(pair):
             params["pair"] = pair
+        if defined(fee_schedule):
+            # Kraken validates this parameter strictly against the JSON
+            # boolean literals "true"/"false" and rejects the default
+            # str(bool) capitalization ("True"/"False") with
+            # EGeneral:Invalid arguments.
+            params["fee_schedule"] = "true" if fee_schedule else "false"
         return self.request(  # type: ignore[return-value]
             method="POST",
             uri="/0/private/TradeVolume",

tests/spot/test_spot_user.py

@@ -53,6 +53,24 @@ def test_get_balance(self: Self) -> None:
                 "available_balance": 1.96307091,
             }
 
+    @pytest.mark.parametrize(
+        ("fee_schedule", "expected"),
+        [(True, "true"), (False, "false")],
+    )
+    def test_get_trade_volume_fee_schedule_serialization(
+        self: Self,
+        fee_schedule: bool,
+        expected: str,
+    ) -> None:
+        """
+        Checks that ``fee_schedule`` is sent as the lowercase JSON boolean
+        literal Kraken expects, not Python's ``str(bool)`` capitalization
+        (which Kraken rejects with ``EGeneral:Invalid arguments``).
+        """
+        with mock.patch.object(User, "request", return_value={}) as request:
+            User().get_trade_volume(fee_schedule=fee_schedule)
+            assert request.call_args.kwargs["params"]["fee_schedule"] == expected
+
 
 @pytest.mark.integration
 @pytest.mark.spot
@@ -290,6 +308,17 @@ def test_get_trade_volume(self: Self, spot_auth_user: User) -> None:
             spot_auth_user.get_trade_volume(pair="DOT/EUR", fee_info=False),
         )
 
+    def test_get_trade_volume_fee_schedule(self: Self, spot_auth_user: User) -> None:
+        """
+        Checks that ``fee_schedule=True`` includes the ``schedules`` field
+        in the response, replacing the deprecated ``fees``/``fees_maker``
+        fields on ``Market.get_asset_pairs``.
+        """
+        response = spot_auth_user.get_trade_volume(pair="XBTUSD", fee_schedule=True)
+        assert is_not_error(response)
+        assert "schedules" in response
+        assert response["schedules"][0]["tiers"][0]["maker_fee"]
+
     @pytest.mark.parametrize("report", ["trades", "ledgers"])
     def test_request_save_export_report(
         self: Self,