fix(langchain): escape empty json braces in prompts

[sjsjsjjs534]

Summary

• Treat bare empty JSON object braces as literal braces when converting Langfuse prompts for LangChain.
• Add a regression test covering {}, { }, and multiline empty-object literals alongside normal LangChain variables.

Fixes langfuse/langfuse#14721

Verification

• RED: UV_CACHE_DIR=.uv-cache uv run --frozen pytest tests/unit/test_prompt_compilation.py::TestLangchainPromptCompilation::test_langchain_prompt_escapes_bare_empty_json_objects -q (failed with LangChain formatting IndexError before the fix)
• UV_CACHE_DIR=.uv-cache uv run --frozen pytest tests/unit/test_prompt_compilation.py::TestLangchainPromptCompilation::test_langchain_prompt_escapes_bare_empty_json_objects -q
• UV_CACHE_DIR=.uv-cache uv run --frozen pytest tests/unit/test_prompt_compilation.py -q
• UV_CACHE_DIR=.uv-cache uv run --frozen ruff check .
• UV_CACHE_DIR=.uv-cache uv run --frozen ruff format --check langfuse/model.py tests/unit/test_prompt_compilation.py
• UV_CACHE_DIR=.uv-cache uv run --frozen mypy langfuse --no-error-summary
• git diff --check

Greptile Summary

This PR fixes a crash when Langfuse prompts containing bare empty JSON objects ({}, { }, or {\n}) are converted for use with LangChain. Previously, an empty {} would reach LangChain as an unescaped single-brace pair, causing an IndexError during .format().

• Fix (langfuse/model.py): A one-character addition — "}" is added to the lookahead set that classifies an opening { as JSON-related, so empty objects are now doubled to {{}} before reaching LangChain's template formatter.
• Regression test: Covers the three empty-object forms ({}, { }, {\n}) mixed with both Langfuse-style ({{test_var}}) and LangChain-style ({langchain_var}) variables in a single prompt.

Confidence Score: 5/5

Safe to merge — the change is minimal and targeted, and the added test directly reproduces the previously crashing scenario.

The logic change is a single-character addition to a lookahead character set inside a well-isolated static method. Adding } to the set that marks an opening brace as JSON-related correctly handles all three empty-object forms. Existing behaviour for non-empty JSON objects and LangChain variable placeholders is unaffected.

No files require special attention.

_{Reviews (1): Last reviewed commit: "fix(langchain): escape empty json braces..." | Re-trigger Greptile}

[claude]

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