Migrate langfuse to v3

[vblagoje]

Why

This PR migrates the Langfuse integration from Langfuse v2 to v3. Langfuse v3 is built on OpenTelemetry (OTel), which differs from v2's trace creation model. The migration introduces an expected extra root span level due to OTel's requirement that trace attributes must be held by a span, unlike v2's direct trace creation capability.
TLDR - you'll see an extra nested root span.

• fixes Update Langfuse integration for Python SDK v3 #1903

Proposed Changes

• Complete migration from Langfuse v2 to v3 API
• User's will not notice any difference in traces (other than the extra root)

How did you test it?

• Updated existing unit tests to work with v3 API
• Adjusted some tests to work with extra root span
• Manually tested in complex multi-agent itinerary setup. Here is the trace. Note the double top nested root span. Here is how the trace looks like in v2 . Hassieb from Lanfguse told me: "exactly, it is expected that there is one root span under the trace, as OTel does not have a concept of a trace that holds attributes (whereas Langfuse does). So the behavior you are seeing is expected"

Notes for the reviewer

The extra root span level is expected behavior in v3 due to OTel's architecture - it's not a bug but a fundamental difference from v2. Some tests needed to be updated to account for this.

[vblagoje]

@LastRemote would you please, at your convenience, try out this PR on your async setup we used to previously test #2207 It would be amazing to nail this transition to v3 without regressions and additional cleanups. 🙏

[sjrl]

@vblagoje just so you are aware I also opened a PR here #2257 that enabled linting on the examples and tests folder. Given that you reworked the langfuse integration in this PR feel free to close mine if it no longer makes sense.