message editor: Only allow types of content the agent can handle (#36616)

Uses the new
[`acp::PromptCapabilities`](a39b7f635d/rust/agent.rs (L194-L215))
to disable non-file mentions and images for agents that don't support
them.

Release Notes:

- N/A

crates/acp_thread/src/connection.rs

@@ -38,6 +38,8 @@ pub trait AgentConnection {
         cx: &mut App,
     ) -> Task<Result<acp::PromptResponse>>;
 
+    fn prompt_capabilities(&self) -> acp::PromptCapabilities;
+
     fn resume(
         &self,
         _session_id: &acp::SessionId,
@@ -334,6 +336,14 @@ mod test_support {
             Task::ready(Ok(thread))
         }
 
+        fn prompt_capabilities(&self) -> acp::PromptCapabilities {
+            acp::PromptCapabilities {
+                image: true,
+                audio: true,
+                embedded_context: true,
+            }
+        }
+
         fn authenticate(
             &self,
             _method_id: acp::AuthMethodId,