fix: preserve structured additionalProperties schemas for Dict types

Addresses review feedback:
- Only set additionalProperties=False when it's True (from extra="allow")
  or missing, preserving structured schemas for Dict[str, T] types
- Convert test to use inline snapshots as requested

This fix ensures that Dict[str, T] typed fields maintain their structured
additionalProperties schemas (e.g., {"type": "number"}) instead of being
forced to False, which would break validation for dynamic keys.

Author: yashwantbezawada

src/openai/lib/_pydantic.py

@@ -48,10 +48,13 @@ def _ensure_strict_json_schema(
 
     typ = json_schema.get("type")
     if typ == "object":
-        # Always set additionalProperties to False for strict schema compliance.
+        # Set additionalProperties to False for strict schema compliance, but preserve
+        # structured schemas (e.g., for Dict[str, T] which needs {"additionalProperties": {schema}}).
         # The OpenAI API requires additionalProperties=false for structured output,
         # even if Pydantic models use extra="allow" which sets it to True.
-        json_schema["additionalProperties"] = False
+        additional_props = json_schema.get("additionalProperties")
+        if additional_props is True or "additionalProperties" not in json_schema:
+            json_schema["additionalProperties"] = False
 
     # object types
     # { 'type': 'object', 'properties': { 'a':  {...} } }

tests/lib/test_pydantic.py

@@ -426,11 +426,12 @@ class MyClassWithExtraAllow(BaseModel):
 
     schema = to_strict_json_schema(MyClassWithExtraAllow)
 
-    # The schema must have additionalProperties set to False
-    assert schema.get("additionalProperties") == False, \
-        "additionalProperties must be False for API compliance, even with extra='allow'"
-
-    # Verify the rest of the schema is correct
-    assert schema["type"] == "object"
-    assert "field" in schema["properties"]
-    assert schema["required"] == ["field"]
+    assert schema == snapshot(
+        {
+            "properties": {"field": {"description": "A test field", "title": "Field", "type": "string"}},
+            "required": ["field"],
+            "title": "MyClassWithExtraAllow",
+            "type": "object",
+            "additionalProperties": False,
+        }
+    )