[bcdonadio]

TL;DR

Create parentless roots for explicit trace_id: values so v4 trace summaries can select input/output.

Why

Depends on #129 for legacy summaries. Explicit roots use root sampling. This differs from Python's placeholder-parent behavior. Applications that replace the ID generator retain the placeholder parent.

Checklist

• Has label
• Has linked issue
• Tests added for new behavior
• Docs updated (if user-facing)

Related to #125. Stack order: #129, then #128.

Verification

Automated tests, RuboCop, and live Cloud CLI/UI checks passed for parentage and input/output. The internal v4 summary query remains unverified.

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Note

Medium Risk
Changes core span parentage, sampling, and context for explicit trace IDs; behavior shifts for apps relying on placeholder parents, though custom ID generators retain the old path.

Overview
Explicit trace_id: on start_observation / observe now starts a parentless OpenTelemetry root (even with an active span), uses the provider root sampler, and restores ambient context when the block ends. Children inside the explicit root still attach to that root; repeated calls with the same ID remain separate roots and still get the legacy as_root mark for v4 trace input/output summaries.

Implementation adds RootSpanIdGenerator on the Langfuse tracer provider and routes explicit roots through with_trace_id, which clears the ambient parent and Langfuse trace baggage claim while scoping the requested trace ID to the span start. otel_tracer_and_id_generator keeps tracer and generator paired if the provider is reconfigured. Replacing Langfuse.tracer_provider.id_generator falls back to the previous placeholder-parent behavior (documented).

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