fix: prevent mutation of original dict when record.msg is a dict

[gaoflow]

Summary

When record.msg is a dict (used as a structured log message), the format() method assigns it to message_dict by reference. Subsequent mutations to message_dict -- such as injecting exc_info or stack_info -- leak into the caller's original dict, causing unexpected side effects.

Reproduction

import logging
from pythonjsonlogger import jsonlogger

logger = logging.getLogger("test")
handler = logging.StreamHandler()
handler.setFormatter(jsonlogger.JsonFormatter())
logger.addHandler(handler)

data = {"key": "value"}
try:
    1 / 0
except:
    logger.exception(data)

# data now contains "exc_info" -- the original dict was mutated!

Fix

Use record.msg.copy() instead of a direct reference, so the original dict remains untouched.

Tests

All 135 existing tests pass.

(This contribution was made under my direction and I take full responsibility for its correctness.)