feat(observability): Wire Observer into gateway runtime

[rexlunae]

Integrates the Observer trait into the gateway's request lifecycle.

Changes

• Add SharedObserver type alias and optional parameter to run_gateway
• Thread observer through handle_connection → dispatch_text_message
• Record events at key lifecycle points:
  • LlmRequest — before provider calls (provider, model, message count)
  • LlmResponse — after calls (duration, success, error message)
  • ToolCallStart — before each tool execution
  • ToolCall — after (duration, success)

Usage

Call sites currently pass None for the observer. To enable observability:

use rustyclaw_core::observability::{LogObserver, Observer};
use std::sync::Arc;

let observer: Arc<dyn Observer> = Arc::new(LogObserver);
run_gateway(..., Some(observer), cancel).await?;

Next Steps

• Wire LogObserver by default when a config flag is set
• Extract token counts from provider responses
• Add ObserverMetric recording (latencies, queue depth)

[rexlunae]

LGTM! Clean integration:

• Observer threaded through the right path (run_gateway → handle_connection → dispatch_text_message)
• Events recorded at correct lifecycle points with timing
• Good TODOs for token extraction

One future enhancement: consider adding ObserverEvent::Error when provider calls fail (in the Err(err) branch around line 1910). But that's minor — this gets the infrastructure in place.

Ship it! 🚢