Keep tail-sampling buffers until every span in the trace has ended

[gyanu2507]

The buffer was being dropped as soon as the root span ended. Anything that finished after that — a detached child, a background task, a streaming finalizer — hit the no-buffer path and was exported even if the sampler had already decided to drop the trace.

The same release also meant a late span that did meet the sampling criteria could no longer pull the rest of the trace in, so the backend only saw a rootless fragment.

This keeps the buffer until every started span has ended. A dropped trace stays dropped; a late qualifying span can still include the whole trace. Matches the approach in pydantic/logfire-js#230.

Closes #2273

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📝 Walkthrough

Walkthrough

Tail sampling now counts started spans for each trace. The trace buffer remains active until all tracked spans end, instead of being removed when the root span ends. Sampled traces can still be released when sampling criteria are met. New tests verify that late spans from dropped traces are not exported and that a late child can cause the complete trace to be exported.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: retaining tail-sampling buffers until all spans in a trace end.
Description check	✅ Passed	The description directly explains the dropped-trace bug, late qualifying spans, and the implemented buffer-lifetime fix.
Linked Issues check	✅ Passed	The implementation tracks outstanding spans and the tests cover both dropped traces and late spans that cause sampling, satisfying issue #2273.
Out of Scope Changes check	✅ Passed	The code and tests remain focused on tail-sampling buffer lifetime and the behavior described in issue #2273.

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[alexmojaki]

Thanks!