agent: Show the usage limits returned from the API (#29236)

This PR updates the usage banners in the Agent panel to use the limits
returned from the API instead of basing it off the plan.

This will allow limits to be updated from the server rather than being
embedded in the client.

Release Notes:

- N/A

crates/agent/src/assistant_panel.rs

@@ -1550,7 +1550,7 @@ impl AssistantPanel {
     fn render_usage_banner(&self, cx: &mut Context<Self>) -> Option<AnyElement> {
         let usage = self.thread.read(cx).last_usage()?;
 
-        Some(UsageBanner::new(zed_llm_client::Plan::ZedProTrial, usage.amount).into_any_element())
+        Some(UsageBanner::new(zed_llm_client::Plan::ZedProTrial, usage).into_any_element())
     }
 
     fn render_last_error(&self, cx: &mut Context<Self>) -> Option<AnyElement> {