client: Raise error for non-JSON-serializable parameters

Fixes issue #69 by providing clear error messages when parameters
cannot be serialized to JSON, instead of silently suppressing the error.

Changes:
- Catch only TypeError instead of all Exceptions when serializing parameters
- Raise TypeError with descriptive message for non-serializable types
- Add test cases for Decimal and custom objects
- Verify None requests still work correctly

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

Author: echarrod

luno_python/base_client.py

@@ -70,10 +70,13 @@ def do(self, method, path, req=None, auth=False):
         :type req: object
         :type auth: bool
         """
-        try:
-            params = json.loads(json.dumps(req))
-        except Exception:
+        if req is None:
             params = None
+        else:
+            try:
+                params = json.loads(json.dumps(req))
+            except TypeError as e:
+                raise TypeError('luno: request parameters must be JSON-serializable: %s' % str(e))
         headers = {'User-Agent': self.make_user_agent()}
         args = dict(timeout=self.timeout, params=params, headers=headers)
         if auth:

tests/test_client.py

@@ -1,6 +1,7 @@
 import pytest
 import requests
 import requests_mock
+from decimal import Decimal
 
 try:
     from json.decoder import JSONDecodeError
@@ -219,3 +220,29 @@ def test_get_balances_with_malformed_response():
     # Test without account_id on malformed response
     result = c.get_balances()
     assert result == mock_response
+
+
+def test_client_do_with_non_serializable_params():
+    """Test that non-JSON-serializable parameters raise a clear error."""
+    c = Client()
+    c.set_base_url('mock://test/')
+
+    # Test with Decimal (not JSON-serializable)
+    with pytest.raises(TypeError) as exc_info:
+        c.do('GET', '/', req={'amount': Decimal('10.5')})
+    assert 'JSON-serializable' in str(exc_info.value)
+
+    # Test with custom object (not JSON-serializable)
+    class CustomObject:
+        pass
+
+    with pytest.raises(TypeError) as exc_info:
+        c.do('GET', '/', req={'obj': CustomObject()})
+    assert 'JSON-serializable' in str(exc_info.value)
+
+    # Test with None request (should not raise)
+    adapter = requests_mock.Adapter()
+    c.session.mount('mock', adapter)
+    adapter.register_uri('GET', 'mock://test/', json={'result': 'ok'})
+    result = c.do('GET', '/', req=None)
+    assert result['result'] == 'ok'