feat(plugins): add BigQuery Agent Analytics delivery and termination observability

Add an unconditional per-row event_id assigned before enqueue so Storage Write
API retry duplicates are identifiable, and add an opt-in exactly_once_delivery
mode that uses one loop-local committed stream with explicit offsets, sticky
ambiguous-send state, an offset_conflict drop bucket, and non-blocking stream
rotation. Expose finish_reason and sanitized error_message on final LLM
responses only, so progressive SSE does not double count. Emit NODE_OUTPUT and
NODE_ERROR for final workflow-node results while keeping model finish and block
diagnostics classified as LLM_RESPONSE. Remove the dead module-level
OpenTelemetry tracer allocation.

Co-authored-by: Haiyuan Cao <haiyuan@google.com>
PiperOrigin-RevId: 962420983

Author: haiyuan-eng-google