fix(tracing): report spans that outlive their sent parent

[solnic]

A span that outlives its transaction root (ie async work via Tasks/Broadway/Oban continuing a trace after the root was reported) was either corrupted or lost:

• If it was still running when the root ended, it was serialized into the root transaction with "timestamp": null — Relay patches that into a zero-duration span with a bogus deadline_exceeded status (first screenshot; the span actually ran for 500ms and succeeded).
• Once it finished, it was silently dropped.

Unfinished children are now excluded from the transaction payload, and sent transaction roots leave a short-lived marker in span storage. A span ending with a marked parent is promoted to a follow-up transaction in the same trace - same trace_id, parent_span_id pointing at the sent root, tagged sentry.parent_span_already_sent: true.

Before

After

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Fixes #1011

[cursor]

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