core: update json output parser (#15079)

- **Description:** changed json.py to handle additional cases of partial
json string to be parsed, basically by dropping the last character in
the string until a valid json string is found or the string is empty.
Also added additional test cases.
  
- **Issue:** function parse_partial_json could not parse cases where the
key is present but the value is not.

---------

Co-authored-by: Nuno Campos <nuno@langchain.dev>

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

@@ -199,6 +199,9 @@ TEST_CASES_PARTIAL = [
     ('{"foo": "bar", "bar": "foo}', '{"foo": "bar", "bar": "foo}"}'),
     ('{"foo": "bar", "bar": "foo[', '{"foo": "bar", "bar": "foo["}'),
     ('{"foo": "bar", "bar": "foo\\"', '{"foo": "bar", "bar": "foo\\""}'),
+    ('{"foo": "bar", "bar":', '{"foo": "bar"}'),
+    ('{"foo": "bar", "bar"', '{"foo": "bar"}'),
+    ('{"foo": "bar", ', '{"foo": "bar"}'),
 ]