bitloi 6499bce2a6 fix: Langfuse chat observation (#15026) ...

### What problem does this PR solve?

Closes #15025

Langfuse-enabled `dialog_service.async_chat()` regressed to
`langfuse_tracer.start_generation(...)` after the earlier Langfuse v4
migration. Langfuse v4 uses `start_observation(as_type="generation")`,
so the remaining `start_generation` call can fail when chat tracing is
enabled.

This restores the migrated `start_observation(as_type="generation")`
call for chat observations while preserving the existing trace context,
model, input payload, and update/end flow. It also adds a regression
test with a fake Langfuse v4-style client that exposes
`start_observation()` but not `start_generation()`.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

### Tests

- `.venv/bin/pytest
test/unit_test/api/db/services/test_dialog_service_final_answer.py -q`
- `.venv/bin/ruff check api/db/services/dialog_service.py
test/unit_test/api/db/services/test_dialog_service_final_answer.py`

2026-05-20 15:01:19 +08:00

..

db

fix: Langfuse chat observation (#15026)

2026-05-20 15:01:19 +08:00

utils

Feature/table parser column roles (#13710)

2026-05-11 10:06:04 +08:00