Switch FX rates to Frankfurter

tests/test_fx.py

@@ -19,7 +19,7 @@ class TestReadCachedRates:
         assert result == {}
 
     def test_read_cached_rates_valid_file(self) -> None:
-        """Test reading valid cached rates file."""
+        """Test reading valid exchangerate.host cached rates file."""
         currencies = ["USD", "EUR", "JPY"]
         data = {
             "quotes": {
@@ -46,6 +46,37 @@ class TestReadCachedRates:
         finally:
             os.unlink(filepath)
 
+    def test_read_cached_rates_frankfurter_file(self) -> None:
+        """Test reading valid Frankfurter cached rates file."""
+        currencies = ["USD", "EUR", "JPY"]
+        data = [
+            {"date": "2026-06-20", "base": "GBP", "quote": "USD", "rate": 1.25},
+            {"date": "2026-06-20", "base": "GBP", "quote": "EUR", "rate": 1.15},
+            {"date": "2026-06-20", "base": "GBP", "quote": "JPY", "rate": 150.0},
+            {
+                "date": "2026-06-20",
+                "base": "GBP",
+                "quote": "CAD",
+                "rate": 1.70,
+            },  # Not requested
+        ]
+
+        with tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False) as f:
+            json.dump(data, f)
+            filepath = f.name
+
+        try:
+            result = read_cached_rates(filepath, currencies)
+
+            assert len(result) == 3
+            assert result["USD"] == Decimal("1.25")
+            assert result["EUR"] == Decimal("1.15")
+            assert result["JPY"] == Decimal("150.0")
+            assert "CAD" not in result  # Not requested
+
+        finally:
+            os.unlink(filepath)
+
     def test_read_cached_rates_missing_currencies(self) -> None:
         """Test with some currencies missing from data."""
         currencies = ["USD", "EUR", "CHF"]  # CHF not in data
@@ -158,8 +189,8 @@ class TestReadCachedRates:
             os.unlink(filepath)
 
 
-def test_get_rates_uses_older_valid_cache_when_latest_attempt_is_usage_limit() -> None:
-    """A usage-limit API response should not replace usable cached rates."""
+def test_get_rates_ignores_old_usage_limit_attempt_when_using_frankfurter() -> None:
+    """An old provider usage-limit response should not block Frankfurter refreshes."""
     with tempfile.TemporaryDirectory() as temp_dir:
         fx_dir = os.path.join(temp_dir, "fx")
         os.mkdir(fx_dir)
@@ -200,8 +231,31 @@ def test_get_rates_uses_older_valid_cache_when_latest_attempt_is_usage_limit() -
         ) as mock_client:
             mock_datetime.now.return_value = datetime(2026, 6, 20, 12, 35)
             mock_datetime.strptime.side_effect = datetime.strptime
+            mock_response = (
+                mock_client.return_value.__enter__.return_value.get.return_value
+            )
+            mock_response.text = json.dumps(
+                [
+                    {
+                        "date": "2026-06-20",
+                        "base": "GBP",
+                        "quote": "EUR",
+                        "rate": 1.16,
+                    },
+                    {
+                        "date": "2026-06-20",
+                        "base": "GBP",
+                        "quote": "USD",
+                        "rate": 1.26,
+                    },
+                ]
+            )
 
             result = get_rates(config)
 
-        assert result == {"USD": Decimal("1.25"), "EUR": Decimal("1.15")}
-        mock_client.assert_not_called()
+        assert result == {"USD": Decimal("1.26"), "EUR": Decimal("1.16")}
+        mock_client.return_value.__enter__.return_value.get.assert_called_once_with(
+            "https://api.frankfurter.dev/v2/rates",
+            params={"base": "GBP", "quotes": "EUR,USD"},
+            timeout=10,
+        )