fix: remove duplicate observability key in pyproject.toml

- Removed duplicate 'observability' section that caused TOML parse errors in CI
- Only one 'observability' section should exist (after 'offline', before 'evaluation')
- Fixes ruff pre-commit hook failure
- Resolves GitHub Actions CI failure on linting step

Author: anouar-bm

pyproject.toml

@@ -127,11 +127,6 @@ evaluation = [
     "pytest-asyncio>=1.2.0",
 ]
 
-observability = [
-    # LLM observability and tracing dependencies
-    "langfuse>=3.8.1",
-]
-
 [project.scripts]
 lightrag-server = "lightrag.api.lightrag_server:main"
 lightrag-gunicorn = "lightrag.api.run_with_gunicorn:main"