adk-2.0 design: OTel correlation strategy (otel_span_id reliability + source_event_id ↔ associated_event_ids)

Parent tracker: #190 (v15 contract).
Wave: 3 — Design blocker.
Confirms or revises #205 (`attributes.adk.otel_span_id`).

## Problem

ADK records `associated_event_ids` on its OTel spans, not span IDs on events. So copying the ambient OTel `span_id` into BQAA rows (#205) may not reliably correlate with the workflow node span. The reliable Cloud-Trace join is `attributes.adk.source_event_id` ↔ ADK's span-side `associated_event_ids`.

## Resolve

Decide:

- Is `attributes.adk.otel_span_id` reliable enough to ship as best-effort (#205), or should it be removed entirely?
- Is the `source_event_id` ↔ `associated_event_ids` join sufficient on the consumer side without a dedicated OTel exporter?
- If a future OTel exporter is needed, file a separate tracking item (do not bundle into #190).

## Acceptance

- [ ] Decision recorded on `otel_span_id` (keep best-effort / remove / promote to required).
- [ ] Consumer-side guidance documented for joining BQAA → Cloud Trace.
- [ ] If a follow-up exporter is needed, a separate tracker is filed.

## References

- #190 (v15 producer §8; v3 OTel correlation downgrade).
- ADK OTel: `src/google/adk/telemetry/` (semconv v1.36 agents, v1.41 workflows).