core: JsonOutputParser UTF characters bug (#27306)

**Description:**
This PR fixes an issue where non-ASCII characters in Pydantic field
descriptions were being escaped to their Unicode representations when
using `JsonOutputParser`. The change allows non-ASCII characters to be
preserved in the output, which is especially important for multilingual
support and when working with non-English languages.

**Issue:** Fixes #27256

**Example Code:**
```python
from pydantic import BaseModel, Field
from langchain_core.output_parsers import JsonOutputParser

class Article(BaseModel):
    title: str = Field(description="科学文章的标题")

output_data_structure = Article
parser = JsonOutputParser(pydantic_object=output_data_structure)
print(parser.get_format_instructions())
```
**Previous Output**:
```... "title": {"description": "\\u79d1\\u5b66\\u6587\\u7ae0\\u7684\\u6807\\u9898", "title": "Title", "type": "string"}} ...```

**Current Output**:
```... "title": {"description": "科学文章的标题", "title": "Title", "type":
"string"}} ...```

**Changes made**:
- Modified `json.dumps()` call in
`langchain_core/output_parsers/json.py` to use `ensure_ascii=False`
- Added a unit test to verify Unicode handling

Co-authored-by: Harsimran-19 <harsimran1869@gmail.com>

libs/core/langchain_core/output_parsers/json.py

@@ -115,7 +115,7 @@ class JsonOutputParser(BaseCumulativeTransformOutputParser[Any]):
             if "type" in reduced_schema:
                 del reduced_schema["type"]
             # Ensure json in context is well-formed with double quotes.
-            schema_str = json.dumps(reduced_schema)
+            schema_str = json.dumps(reduced_schema, ensure_ascii=False)
             return JSON_FORMAT_INSTRUCTIONS.format(schema=schema_str)
 
     @property

libs/core/tests/unit_tests/output_parsers/test_json.py

@@ -3,7 +3,7 @@ from collections.abc import AsyncIterator, Iterator
 from typing import Any
 
 import pytest
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 
 from langchain_core.exceptions import OutputParserException
 from langchain_core.output_parsers.json import (
@@ -603,3 +603,16 @@ def test_base_model_schema_consistency() -> None:
 
     assert initial_joke_schema == retrieved_joke_schema
     assert openai_func.get("name", None) is not None
+
+
+def test_unicode_handling() -> None:
+    """Tests if the JsonOutputParser is able to process unicodes."""
+
+    class Sample(BaseModel):
+        title: str = Field(description="科学文章的标题")
+
+    parser = SimpleJsonOutputParser(pydantic_object=Sample)
+    format_instructions = parser.get_format_instructions()
+    assert (
+        "科学文章的标题" in format_instructions
+    ), "Unicode characters should not be escaped"