feat(mixer): enhance mixer functionality with interaction and tool legs

- Updated mixer to handle participant and isolated leg roles, allowing for IVR and consent interactions.
- Introduced commands for starting and canceling interactions, managing tool legs for recording and transcription.
- Implemented per-source audio handling for tool legs, enabling separate audio processing.
- Enhanced DTMF handling to forward events between participant legs only.
- Added support for PCM recording directly from tool legs, with WAV file generation.
- Updated TypeScript definitions and functions to support new interaction and tool leg features.

rust/crates/proxy-engine/src/provider.rs

@@ -321,6 +321,17 @@ impl ProviderManager {
         None
     }
 
+    /// Find a provider by its config ID (e.g. "easybell").
+    pub async fn find_by_provider_id(&self, provider_id: &str) -> Option<Arc<Mutex<ProviderState>>> {
+        for ps_arc in &self.providers {
+            let ps = ps_arc.lock().await;
+            if ps.config.id == provider_id {
+                return Some(ps_arc.clone());
+            }
+        }
+        None
+    }
+
     /// Check if a provider is currently registered.
     pub async fn is_registered(&self, provider_id: &str) -> bool {
         for ps_arc in &self.providers {