agent: Add "max mode" toggle (#29549)

This PR adds a "max mode" toggle to the Agent panel, for models that
support it.

Only visible to folks in the `new-billing` feature flag.

Icon is just a placeholder.

Release Notes:

- N/A

crates/agent/src/thread.rs

@@ -34,6 +34,7 @@ use settings::Settings;
 use thiserror::Error;
 use util::{ResultExt as _, TryFutureExt as _, post_inc};
 use uuid::Uuid;
+use zed_llm_client::CompletionMode;
 
 use crate::context::{AgentContext, ContextLoadResult, LoadedContext};
 use crate::thread_store::{
@@ -290,6 +291,7 @@ pub struct Thread {
     summary: Option<SharedString>,
     pending_summary: Task<Option<()>>,
     detailed_summary_state: DetailedSummaryState,
+    completion_mode: Option<CompletionMode>,
     messages: Vec<Message>,
     next_message_id: MessageId,
     last_prompt_id: PromptId,
@@ -339,6 +341,7 @@ impl Thread {
             summary: None,
             pending_summary: Task::ready(None),
             detailed_summary_state: DetailedSummaryState::NotGenerated,
+            completion_mode: None,
             messages: Vec::new(),
             next_message_id: MessageId(0),
             last_prompt_id: PromptId::new(),
@@ -394,6 +397,7 @@ impl Thread {
             summary: Some(serialized.summary),
             pending_summary: Task::ready(None),
             detailed_summary_state: serialized.detailed_summary_state,
+            completion_mode: None,
             messages: serialized
                 .messages
                 .into_iter()
@@ -518,6 +522,14 @@ impl Thread {
         }
     }
 
+    pub fn completion_mode(&self) -> Option<CompletionMode> {
+        self.completion_mode
+    }
+
+    pub fn set_completion_mode(&mut self, mode: Option<CompletionMode>) {
+        self.completion_mode = mode;
+    }
+
     pub fn message(&self, id: MessageId) -> Option<&Message> {
         let index = self
             .messages
@@ -930,6 +942,12 @@ impl Thread {
         self.remaining_turns -= 1;
 
         let mut request = self.to_completion_request(cx);
+        request.mode = if model.supports_max_mode() {
+            self.completion_mode
+        } else {
+            None
+        };
+
         if model.supports_tools() {
             request.tools = self
                 .tools()
@@ -967,6 +985,7 @@ impl Thread {
         let mut request = LanguageModelRequest {
             thread_id: Some(self.id.to_string()),
             prompt_id: Some(self.last_prompt_id.to_string()),
+            mode: None,
             messages: vec![],
             tools: Vec::new(),
             stop: Vec::new(),
@@ -1063,6 +1082,7 @@ impl Thread {
         let mut request = LanguageModelRequest {
             thread_id: None,
             prompt_id: None,
+            mode: None,
             messages: vec![],
             tools: Vec::new(),
             stop: Vec::new(),