acp: Model-specific prompt capabilities for 1PA (#36879)

Adds support for per-session prompt capabilities and capability changes
on the Zed side (ACP itself still only has per-connection static
capabilities for now), and uses it to reflect image support accurately
in 1PA threads based on the currently-selected model.

Release Notes:

- N/A

crates/agent_servers/src/acp.rs

@@ -185,13 +185,16 @@ impl AgentConnection for AcpConnection {
 
             let session_id = response.session_id;
             let action_log = cx.new(|_| ActionLog::new(project.clone()))?;
-            let thread = cx.new(|_cx| {
+            let thread = cx.new(|cx| {
                 AcpThread::new(
                     self.server_name.clone(),
                     self.clone(),
                     project,
                     action_log,
                     session_id.clone(),
+                    // ACP doesn't currently support per-session prompt capabilities or changing capabilities dynamically.
+                    watch::Receiver::constant(self.prompt_capabilities),
+                    cx,
                 )
             })?;
 
@@ -279,10 +282,6 @@ impl AgentConnection for AcpConnection {
         })
     }
 
-    fn prompt_capabilities(&self) -> acp::PromptCapabilities {
-        self.prompt_capabilities
-    }
-
     fn cancel(&self, session_id: &acp::SessionId, cx: &mut App) {
         if let Some(session) = self.sessions.borrow_mut().get_mut(session_id) {
             session.suppress_abort_err = true;

crates/agent_servers/src/claude.rs

@@ -249,13 +249,19 @@ impl AgentConnection for ClaudeAgentConnection {
             });
 
             let action_log = cx.new(|_| ActionLog::new(project.clone()))?;
-            let thread = cx.new(|_cx| {
+            let thread = cx.new(|cx| {
                 AcpThread::new(
                     "Claude Code",
                     self.clone(),
                     project,
                     action_log,
                     session_id.clone(),
+                    watch::Receiver::constant(acp::PromptCapabilities {
+                        image: true,
+                        audio: false,
+                        embedded_context: true,
+                    }),
+                    cx,
                 )
             })?;
 
@@ -319,14 +325,6 @@ impl AgentConnection for ClaudeAgentConnection {
         cx.foreground_executor().spawn(async move { end_rx.await? })
     }
 
-    fn prompt_capabilities(&self) -> acp::PromptCapabilities {
-        acp::PromptCapabilities {
-            image: true,
-            audio: false,
-            embedded_context: true,
-        }
-    }
-
     fn cancel(&self, session_id: &acp::SessionId, _cx: &mut App) {
         let sessions = self.sessions.borrow();
         let Some(session) = sessions.get(session_id) else {