Describe the bug

The heuristic in BasePromptClient._escape_json_for_langchain only treats a { as JSON when the next non-whitespace character is a quote, so empty braces {} (and { }) are left unescaped and still break LangChain templates.

Input to ChatPromptTemplate is missing variables {''}. Expected: ['', 'changes_description', ...]

Empty braces are common in prompts that document JSON option fields must be a dict (can be empty {})

Steps to reproduce

from string import Formatter
from langfuse.model import BasePromptClient

for text in ['can be empty {}', 'spaced { } too', '{"col": "x"}']:
out = BasePromptClient._get_langchain_prompt_string(text)
vars = [f for _, f, _, _ in Formatter().parse(out) if f is not None]
print(f"{text!r} -> {out!r} -> template vars {vars}")

'# 'can be empty {}' -> 'can be empty {}' -> [''] ← breaks ChatPromptTemplate
'# 'spaced { } too' -> 'spaced { } too' -> [' '] ← breaks ChatPromptTemplate
'# '{"col": "x"}' -> '{{"col": "x"}}' -> [] ← correctly escaped

Langfuse Cloud or self-hosted?

Self-hosted

If self-hosted, what version are you running?

4.9.0

SDK and integration versions

No response

Additional information

No response

Are you interested in contributing a fix for this bug?

Yes