fix(transfer): don't coerce malformed falsy asset values to XLM

Addresses CodeRabbit review on #50: the asset default validator only
covers None and "" now, so False/0 correctly fail type validation
instead of being silently normalized to "XLM".

Author: angelraph

src/shade/models/transfer.py

@@ -49,10 +49,14 @@ class Transfer(ShadeObject):
 
     @field_validator("asset", mode="before")
     @classmethod
-    def _default_asset(cls, value: Optional[str]) -> str:
+    def _default_asset(cls, value: object) -> object:
         # Covers both a missing key (pydantic would already default it) and an
-        # API response that sends the key with an explicit null/empty value.
-        return value if value else "XLM"
+        # API response that sends the key as an explicit null/empty string.
+        # Other falsy-but-wrong types (e.g. False, 0) are left alone so
+        # pydantic's normal type validation rejects them.
+        if value is None or value == "":
+            return "XLM"
+        return value
 
     @field_validator("source_address", "destination_address")
     @classmethod

tests/test_transfer.py

@@ -60,6 +60,14 @@ def test_asset_defaults_to_xlm_when_null():
     assert transfer.asset == "XLM"
 
 
+def test_malformed_asset_is_rejected_not_defaulted():
+    # False/0 are falsy but must not be silently coerced to "XLM" — they are
+    # the wrong type and should surface as a validation error instead.
+    with pytest.raises(InvalidRequestError) as exc_info:
+        Transfer.from_dict(_api_response(asset=False))
+    assert exc_info.value.param == "asset"
+
+
 def test_status_is_transfer_status_enum():
     for raw in ("pending", "processing", "completed", "failed"):
         transfer = Transfer.from_dict(_api_response(status=raw))