agent: Show request usage in the panel (#29006)

This PR adds a banner showing request usage in the Agent panel:

<img width="640" alt="Screenshot 2025-04-17 at 5 51 46 PM"
src="https://github.com/user-attachments/assets/e0eb036c-57c1-441c-bbab-7dab1c6e56d9"
/>

Only visible to users on the new billing.

Note to Joseph: Doesn't need to be cherry-picked to Preview.

Release Notes:

- N/A

---------

Co-authored-by: Nate <nate@zed.dev>

crates/agent/src/active_thread.rs

@@ -23,7 +23,8 @@ use gpui::{
 };
 use language::{Buffer, LanguageRegistry};
 use language_model::{
-    LanguageModelRegistry, LanguageModelRequestMessage, LanguageModelToolUseId, Role, StopReason,
+    LanguageModelRegistry, LanguageModelRequestMessage, LanguageModelToolUseId, RequestUsage, Role,
+    StopReason,
 };
 use markdown::parser::{CodeBlockKind, CodeBlockMetadata};
 use markdown::{HeadingLevelStyles, Markdown, MarkdownElement, MarkdownStyle, ParsedMarkdown};
@@ -63,6 +64,7 @@ pub struct ActiveThread {
     expanded_thinking_segments: HashMap<(MessageId, usize), bool>,
     expanded_code_blocks: HashMap<(MessageId, usize), bool>,
     last_error: Option<ThreadError>,
+    last_usage: Option<RequestUsage>,
     notifications: Vec<WindowHandle<AgentNotification>>,
     copied_code_block_ids: HashSet<(MessageId, usize)>,
     _subscriptions: Vec<Subscription>,
@@ -734,6 +736,7 @@ impl ActiveThread {
             hide_scrollbar_task: None,
             editing_message: None,
             last_error: None,
+            last_usage: None,
             copied_code_block_ids: HashSet::default(),
             notifications: Vec::new(),
             _subscriptions: subscriptions,
@@ -792,6 +795,10 @@ impl ActiveThread {
         self.last_error.take();
     }
 
+    pub fn last_usage(&self) -> Option<RequestUsage> {
+        self.last_usage
+    }
+
     /// Returns the editing message id and the estimated token count in the content
     pub fn editing_message_id(&self) -> Option<(MessageId, usize)> {
         self.editing_message
@@ -876,6 +883,9 @@ impl ActiveThread {
             ThreadEvent::ShowError(error) => {
                 self.last_error = Some(error.clone());
             }
+            ThreadEvent::UsageUpdated(usage) => {
+                self.last_usage = Some(*usage);
+            }
             ThreadEvent::StreamedCompletion
             | ThreadEvent::SummaryGenerated
             | ThreadEvent::SummaryChanged => {