[BUG] formatting that includes escaped characters seems to fail

The user message contains an escaped character, causing a failure to format the prompt in the Python client.

```python
import phoenix.client as px_cli
px_cli_client = px_cli.Client()

prompt = px_cli_client.prompts.get(
    prompt_identifier="receiptintel",
    prompt_version_id="UHJvbXB0VmVyc2lvbjo4MA==",
)

prompt.format(variables={"user_msg": "2025\l6300"})
```