agent: Add sound notification when done generating (#31472)

This PR adds the ability to hear a sound notification when the agent is
done generating and/or needs user input. This setting is turned off by
default and can be used together with the visual notification. The
specific sound I'm using here comes from the [Material Design 2 Sound
Library](https://m2.material.io/design/sound/sound-resources.html#).

Release Notes:

- agent: Added the ability to have a sound notification when the agent
is done generating and/or needs user input.

crates/audio/src/audio.rs

@@ -18,6 +18,7 @@ pub enum Sound {
     Unmute,
     StartScreenshare,
     StopScreenshare,
+    AgentDone,
 }
 
 impl Sound {
@@ -29,6 +30,7 @@ impl Sound {
             Self::Unmute => "unmute",
             Self::StartScreenshare => "start_screenshare",
             Self::StopScreenshare => "stop_screenshare",
+            Self::AgentDone => "agent_done",
         }
     }
 }