feat: BigQuery Agent Analytics reliability fixes

Three related reliability/observability fixes to the BigQuery Agent
Analytics plugin.

1. Dropped-event observability. BigQuery logging is best-effort: events
   are dropped when the in-memory queue overflows or a write ultimately
   fails, and only a log line records the loss. Track dropped rows in
   BatchProcessor by reason (queue_full, arrow_prep_failed,
   retry_exhausted, non_retryable, unexpected_error), include the
   running total in each drop log line, and expose the counts via
   BatchProcessor.get_drop_stats()/dropped_event_count and an
   aggregating BigQueryAgentAnalyticsPlugin.get_drop_stats() so a host
   can poll them and export to its own monitoring.

2. Cross-region Storage Write API routing. The AppendRows streaming RPC
   does not auto-populate the request-routing header, so writes to a
   dataset outside the US multiregion could fail with a "session not
   found" / stream-not-found error and silently drop every row. Set
   x-goog-request-params: write_stream=<stream> on the append_rows call
   so the request reaches the region that owns the write stream.
   US-multiregion behavior is unchanged.

3. Stop exporting plugin-owned OTel spans. When Agent Engine telemetry
   is enabled (GOOGLE_CLOUD_AGENT_ENGINE_ENABLE_TELEMETRY=true) with
   Cloud Trace export on the global tracer provider, the plugin's
   ID-carrier spans were exported alongside the framework's real spans,
   producing a duplicate span for every instrumented operation. The
   plugin now tracks span_id / trace_id on its own contextvar stack
   without creating OTel spans; trace_id is inherited from the ambient
   span, so BigQuery rows still join to Cloud Trace by trace_id and the
   LLM/tool span_id-sharing contracts are preserved.

All paths covered by unit tests.

Change-Id: Ia7b73d816b14c574ef856a4c88c57243f6f38f7f

Author: haiyuan-eng-google