Fix Json strategy serialization failure

When generating a pydantic model having nested Json fields with
hypothesis, a JSON serialization exception was raised.

pydantic/_hypothesis_plugin.py

@@ -27,7 +27,7 @@
 import json
 import math
 from fractions import Fraction
-from typing import Callable, Dict, Type, Union, cast, overload
+from typing import Any, Callable, Dict, Type, Union, cast, overload
 
 import hypothesis.strategies as st
 
@@ -212,6 +212,15 @@ def inner(f):  # type: ignore
 # Type-to-strategy resolver functions
 
 
+def is_base_model(cls: Any) -> bool:
+    """Returns True if the given class is a subclass of the pydantic BaseModel."""
+
+    try:
+        return issubclass(cls, pydantic.BaseModel)
+    except TypeError:
+        return False
+
+
 @resolves(pydantic.JsonWrapper)
 def resolve_json(cls):  # type: ignore[no-untyped-def]
     try:
@@ -223,7 +232,7 @@ def resolve_json(cls):  # type: ignore[no-untyped-def]
             extend=lambda x: st.lists(x) | st.dictionaries(st.text(), x),  # type: ignore
         )
     return st.builds(
-        json.dumps,
+        cls.inner_type.json if is_base_model(getattr(cls, 'inner_type', None)) else json.dumps,
         inner,
         ensure_ascii=st.booleans(),
         indent=st.none() | st.integers(0, 16),

tests/test_hypothesis_plugin.py

@@ -67,6 +67,7 @@ class JsonModel(pydantic.BaseModel):
         json_str: pydantic.Json[str]
         json_int_or_str: pydantic.Json[typing.Union[int, str]]
         json_list_of_float: pydantic.Json[typing.List[float]]
+        json_pydantic_model: pydantic.Json[pydantic.BaseModel]
 
     class ConstrainedNumbersModel(pydantic.BaseModel):
         conintt: pydantic.conint(gt=10, lt=100)