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_ui/src/acp/message_editor.rs

@@ -373,7 +373,7 @@ impl MessageEditor {
 
         if Img::extensions().contains(&extension) && !extension.contains("svg") {
             if !self.prompt_capabilities.get().image {
-                return Task::ready(Err(anyhow!("This agent does not support images yet")));
+                return Task::ready(Err(anyhow!("This model does not support images yet")));
             }
             let task = self
                 .project

crates/agent_ui/src/acp/thread_view.rs

@@ -474,7 +474,7 @@ impl AcpThreadView {
                         let action_log = thread.read(cx).action_log().clone();
 
                         this.prompt_capabilities
-                            .set(connection.prompt_capabilities());
+                            .set(thread.read(cx).prompt_capabilities());
 
                         let count = thread.read(cx).entries().len();
                         this.list_state.splice(0..0, count);
@@ -1163,6 +1163,10 @@ impl AcpThreadView {
                     });
                 }
             }
+            AcpThreadEvent::PromptCapabilitiesUpdated => {
+                self.prompt_capabilities
+                    .set(thread.read(cx).prompt_capabilities());
+            }
             AcpThreadEvent::TokenUsageUpdated => {}
         }
         cx.notify();
@@ -5367,6 +5371,12 @@ pub(crate) mod tests {
                     project,
                     action_log,
                     SessionId("test".into()),
+                    watch::Receiver::constant(acp::PromptCapabilities {
+                        image: true,
+                        audio: true,
+                        embedded_context: true,
+                    }),
+                    cx,
                 )
             })))
         }
@@ -5375,14 +5385,6 @@ pub(crate) mod tests {
             &[]
         }
 
-        fn prompt_capabilities(&self) -> acp::PromptCapabilities {
-            acp::PromptCapabilities {
-                image: true,
-                audio: true,
-                embedded_context: true,
-            }
-        }
-
         fn authenticate(
             &self,
             _method_id: acp::AuthMethodId,

crates/agent_ui/src/agent_diff.rs

@@ -1529,6 +1529,7 @@ impl AgentDiff {
             | AcpThreadEvent::TokenUsageUpdated
             | AcpThreadEvent::EntriesRemoved(_)
             | AcpThreadEvent::ToolAuthorizationRequired
+            | AcpThreadEvent::PromptCapabilitiesUpdated
             | AcpThreadEvent::Retry(_) => {}
         }
     }