feat: export ADK agent engine template logs to the Telemetry API

[copybara-service]

feat: export ADK agent engine template logs to the Telemetry API

CloudLoggingExporter is deprecated, so the ADK agent engine templates
now send logs over OTLP to telemetry.googleapis.com, alongside the
traces they already export there. Applied to all four copies of the
template (vertexai/agent_engines, agentplatform/agent_engines,
vertexai/preview/reasoning_engines, agentplatform/private/frameworks) so
their telemetry does not diverge.

This collapses the semconv split. Experimental semconv used a batching
processor writing through the Cloud Logging API, while stable semconv
wrote structured JSON to stdout, because Agent Engine mis-parsed
gen_ai.client.inference.operation.details records off stdout. Neither
branch survives the move to OTLP -- there is one path now, and it
behaves the same under both settings.

Because the OTLP mapping is done server side, two behaviours had to be
reproduced explicitly:

• _create_otel_resource(project_id, "logs") pins gcp.resource_type
  and the location/reasoning_engine_id labels, so entries keep landing
  on aiplatform.googleapis.com/ReasoningEngine instead of being
  detected as generic_task. It returns a logs-only resource: the
  metrics pipeline reads gcp.resource_type too, and would move Agent
  Engine metrics off prometheus_target if it saw it.

• _named_batch_log_record_processor keeps GCP_DEFAULT_LOG_NAME (and
  the adk-on-agent-engine default) working, and re-publishes the
  record's event name as an event.name attribute so it still lands
  as a log entry label.

The User-Agent string is now built by _get_user_agent rather than
inline in the tracing branch, so both signals report the same one.

Nothing imports opentelemetry-exporter-gcp-logging or
google-cloud-logging any more -- the ADK templates were their only
users, and the other templates never set up logging -- so both are
dropped from the agent_engines and reasoning_engine extras, and the
missing-dependency warning now names the packages logging actually needs.