fix(langchain): deregister langfuse prompts when a chain run errors

[anishgurjar]

What does this PR do?

on_chain_end deregisters the run's entry in _prompt_to_parent_run_map, but on_chain_error does not. A chain run that errors after a Langfuse prompt was registered for it - a nested run that inherited a registration, or a run whose child prompt chain registered one keyed to it - leaks the entry permanently. In a service that retries failing chains, the map grows without bound.

on_chain_error now deregisters the run in its finally block, so the cleanup runs even when the error update itself raises. This matches the deregistration on_chain_end performs on the success path. Same error-path cleanup class as #1978.

Type of change

• Bug fix
• New feature
• Breaking change
• Refactor
• Documentation update
• Tooling, CI, or repo maintenance

Verification

uv run --frozen pytest tests/unit/test_langchain.py -q   # 21 passed, 5 skipped
uv run --frozen pytest -n auto --dist worksteal tests/unit   # 928 passed, 7 skipped (test_prompt.py errors pre-existing, see #1976)
uv run --frozen ruff check langfuse/langchain/CallbackHandler.py tests/unit/test_langchain.py
uv run --frozen ruff format --check langfuse/langchain/CallbackHandler.py tests/unit/test_langchain.py
uv run --frozen mypy langfuse --no-error-summary   # only pre-existing langgraph stub errors (optional dep)

Both new tests fail on main (the registration survives on_chain_error) and pass with this change.

Checklist

• I self-reviewed the diff using code_review.md.
• I added or updated tests for behavior changes.
• I updated docs, examples, or .env.template if needed. (not needed)
• I did not hand-edit generated files; if generated files changed, I used the upstream regeneration path.
• I did not commit secrets or credentials.

Confidence Score: 5/5

The PR appears safe to merge.

Summary

Removes a failed chain’s prompt registration in on_chain_error, including when updating the observation raises.

• Failed LangChain chains now clear their own prompt registrations.

_{Reviews (1) · Last reviewed commit: "style: trim trailing blank line for ruff..." · Reviewed by Greptile}

[claude]

Claude Code Review

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[anishgurjar]

@hassiebp This is the same error-path cleanup class as #1978, one level up in the prompt-linkage map. Both regression tests fail on main and pass here. Happy to adjust if you would rather see the deregistration shared between the end and error paths.

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