Add OpenTelemetry tracing to the code-mode host (#40760)

## What changed

- Add `--otel-trace-exporter` for sending OTLP/HTTP JSON traces directly to an exporter.
- Add the mutually exclusive `--otel-trace-listen` option for streaming raw OTLP trace batches over a trace-only WebSocket, backed by a loopback OTLP receiver.
- Instrument host startup, transport requests, and gRPC operations with request, session, execution, cell, wait, and tool-call identifiers where available.
- Flush the trace provider with a bounded timeout during shutdown.

GitOrigin-RevId: 4dcdb2bc3b15166dccd5041fb875caea74f3f6fe