Forward exclude_defaults when jsonable_encoder recurses into dict values

jsonable_encoder passes exclude_defaults to its recursive calls for
list/tuple/set items, but omits it for dict keys and values. As a result
a Pydantic model nested inside a dict keeps its default-valued fields
when exclude_defaults=True, while the same model inside a list correctly
drops them:

    jsonable_encoder([m], exclude_defaults=True)        -> [{'foo': 'foo'}]          # correct
    jsonable_encoder({'m': m}, exclude_defaults=True)   -> {'m': {'foo': 'foo', ...}}  # wrong, defaults kept

The sibling flags exclude_unset and exclude_none are already forwarded in
the same dict branch, so this is an inconsistency, not intended behavior.
Forward exclude_defaults too. Adds a regression test.

Authored with the assistance of an AI tool; reviewed and verified by me.

fastapi/encoders.py

@@ -299,6 +299,7 @@ def jsonable_encoder(
                     key,
                     by_alias=by_alias,
                     exclude_unset=exclude_unset,
+                    exclude_defaults=exclude_defaults,
                     exclude_none=exclude_none,
                     custom_encoder=custom_encoder,
                     sqlalchemy_safe=sqlalchemy_safe,
@@ -307,6 +308,7 @@ def jsonable_encoder(
                     value,
                     by_alias=by_alias,
                     exclude_unset=exclude_unset,
+                    exclude_defaults=exclude_defaults,
                     exclude_none=exclude_none,
                     custom_encoder=custom_encoder,
                     sqlalchemy_safe=sqlalchemy_safe,

tests/test_jsonable_encoder.py

@@ -202,6 +202,19 @@ def test_encode_model_with_default():
     }
 
 
+def test_encode_model_with_default_in_container():
+    # exclude_defaults must be forwarded when recursing into dict values, just
+    # like it already is for list/tuple/set items.
+    model = ModelWithDefault(foo="foo", bar="bar")
+    assert jsonable_encoder([model], exclude_defaults=True) == [{"foo": "foo"}]
+    assert jsonable_encoder({"m": model}, exclude_defaults=True) == {
+        "m": {"foo": "foo"}
+    }
+    assert jsonable_encoder({"m": model}, exclude_unset=True) == {
+        "m": {"foo": "foo", "bar": "bar"}
+    }
+
+
 def test_custom_encoders():
     class safe_datetime(datetime):
         pass