fix: Improve BigQuery Agent Analytics plugin reliability and code quality

This CL fixes several bugs in the BigQuery Agent Analytics plugin and refactors the internal data-passing pattern for better type safety and maintainability.

-   **Stale Loop State Validation:** Use `loop.is_closed()` — a public, reliable API — to detect and clean up stale asyncio loop states in `_batch_processor_prop`, `_get_loop_state`, and `flush`. The previous approach used `asyncio.Queue._loop` which is `None` on Python 3.10+, causing the check to always treat states as stale.
-   **Quota Project ID Fallback:** Remove the `or project_id` fallback when setting `quota_project_id` on `BigQueryWriteAsyncClient`. This fixes Workload Identity Federation flows where the federated identity lacks `serviceusage.services.use` on the quota project.
-   **Kwargs Passthrough:** Pass `**kwargs` through to `_log_event` in all callbacks. Previously only model callbacks forwarded them, causing custom attributes (e.g. `customer_id`) to silently drop for agent, tool, run, and error events.
-   **State Delta Logging:** Replace the dead `on_state_change_callback` (never invoked by the framework) with `on_event_callback`, which is already dispatched by the runner for every event. Remove duplicate `STATE_DELTA` logging from `after_tool_callback`.
-   **EventData Dataclass:** Replace the `**kwargs`-as-data-bus pattern in `_log_event` with an explicit `EventData` dataclass. This makes the interface self-documenting, catches typos at construction time, and eliminates shared dict mutation across `_resolve_span_ids`, `_extract_latency`, and `_enrich_attributes`. All 12 callback call sites now construct typed `EventData` instances.
-   **Multi-Subagent Tool Logging Tests:** Add `TestMultiSubagentToolLogging` (6 tests) verifying that tool events are correctly attributed to subagents in multi-turn, multi-agent scenarios. Total tests: 111 (up from 60).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: caohy1988