refactor(plugins)!: use OpenTelemetry for BigQuery plugin tracing

This refactors the BigQueryAgentAnalyticsPlugin to use the standard OpenTelemetry API for trace and span ID generation and propagation, replacing the custom ContextVar implementation.

Key changes:
- Utilizes `opentelemetry.trace` for starting/ending spans.
- Correctly uses `opentelemetry.context` for context attachment and detachment.
- Span information is now derived from the OpenTelemetry context when available.
- Added a fallback mechanism to ensure span_id and parent_span_id are still populated if the OpenTelemetry SDK is not initialized.

To get standard OpenTelemetry trace information in BigQuery logs, users should install `opentelemetry-sdk` and initialize a global `TracerProvider` in their application *before* initializing ADK components.

Example minimal initialization:
```python
# Install: pip install opentelemetry-sdk
from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider
trace.set_tracer_provider(TracerProvider())
```

PiperOrigin-RevId: 858965562

Author: google-genai-bot