Upgrade Langfuse tracing integration to Python SDK v4

Hi Langflow team — I'm Hassieb from the Langfuse team.

Langfuse Cloud's Fast Preview is observations-first. For real-time data in the new UI, Python integrations should use SDK v4.7.0+.

I reviewed services/tracing/langfuse.py; this looks like a focused v3→v4 migration:

• Bump the optional dependency from langfuse~=3.8 to a v4-compatible range such as langfuse>=4.7,<5.
• Rename root and child start_span calls to start_observation.
• Replace the root .update_trace(...) calls with propagate_attributes() around observation creation so trace name, user, session, and metadata are present on every child span.
• Re-run the custom LangChain root-LLM reparenting and feedback/score tests to confirm the tree remains connected under v4.
• Keep the isolated TracerProvider setup and verify its flush/shutdown behavior.

The relevant guide is Python SDK v3→v4.

If you need support with the migration, we're happy to jump on a call or help review a PR.