Upgrade @voltagent/langfuse-exporter to Langfuse JS SDK v5

[hassiebp]

Hi VoltAgent team 👋 — I'm Hassieb from the Langfuse team.

I reviewed @voltagent/langfuse-exporter. It currently groups OTel spans and recreates them through the v3 langfuse event API. With JS/TS SDK v5.4+, this can become a much thinner OTel integration:

• Replace the custom v3-backed exporter with a wrapper around LangfuseSpanProcessor from @langfuse/otel.
• Normalize VoltAgent's custom ai.*/usage.* fields to standard gen_ai.* and Langfuse observation attributes before export.
• If VoltAgent's instrumentation scope is not covered by the v5 default filter, add a scoped shouldExportSpan rule that includes VoltAgent spans without pulling in unrelated HTTP/database spans.
• Attach user/session/tags/trace-name attributes to every span rather than reconstructing them only on the root trace.
• Preserve parent IDs, generation usage/TTFT, error status, batching, force-flush, and shutdown in tests.

Guides: JS SDK v3→v4 and v4→v5.

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