telemetry: don't record error on chat span after successful LLM completion

[jedp-docker]

Summary

• handleStream returns the moment it sees a terminal finish_reason (a deliberate latency optimisation — don't block the next turn on trailing message_stop/EOF). The deferred stream.Close() then closes the HTTP/2 body while the reader goroutine is still blocked in Recv(), causing that read to return "http2: response body closed" (or "context canceled" if the per-attempt streamCancel fires first).
• instrumentedStream was recording this transport-level teardown as a span error even when the completion had fully succeeded — every affected span also carries gen_ai.response.finish_reasons=["stop"] and full token usage, confirming the response was complete.
• This inflates the docker-agent error rate on every LLM call. Observed consistently across all harbor_watch.investigation traces in Tempo.

Fix: add finishReasonSeen bool to instrumentedStream (set-once on the single-consumer Recv path). Once a terminal finish reason is delivered, skip RecordError for subsequent transport errors — they're benign teardown, not failures. endOnce is still called so the span closes.

Test plan

• Verify existing genai package tests pass: go test ./pkg/telemetry/genai/...
• In Tempo: after deploy, confirm chat claude-* spans with finish_reasons=stop no longer appear as STATUS_CODE_ERROR
• Confirm spans that fail before a finish reason (genuine mid-stream errors) still record errors

🤖 Generated with Claude Code

[docker-agent]

Assessment: 🟢 APPROVE

The fix is correct and well-scoped. The finishReasonSeen flag is set on the single-consumer Recv path — no concurrency risk — and the teardown-suppression logic correctly gates only RecordError, leaving endOnce() unconditional so spans always close. All terminal finish reasons are covered (the flag trips on any non-empty FinishReason, not just "stop"). The explanatory comment accurately describes the transport-teardown scenario and the intended invariant.

No bugs found in the introduced code.