runner: start a span before BeforeRunCallback so plugin rows share the invocation's trace #1884

ADK Go opens its first OpenTelemetry span in agent.Run (agent.go#L163). That happens after the runner has already called OnUserMessageCallback and BeforeRunCallback (runner.go#L702-L714), so rows a plugin writes in those two callbacks can't carry the trace ID of ADK's spans.

With #1787 and ADK telemetry on, the BigQuery Agent Analytics plugin writes USER_MESSAGE_RECEIVED and INVOCATION_STARTING under one trace ID and every later row of the same invocation under ADK's (trace_manager.go#L319-L337). The same span_id (the invocation span) therefore appears under two trace IDs, and anything that rebuilds a trace by trace_id splits the invocation in two.

adk-python avoids this because its runner's invocation span already exists when the plugin starts, which keeps "every event in the invocation" on one trace ID (bigquery_agent_analytics_plugin.py#L2862-L2868, #L2897-L2900).

A fix should give every plugin callback of one invocation the same trace when ADK telemetry is on.