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

Uses the new
[`acp::PromptCapabilities`](https://github.com/zed-industries/agent-client-protocol/blob/a39b7f635d67528f0a4e05e086ab283b9fc5cb93/rust/agent.rs#L194-L215)
to disable non-file mentions and images for agents that don't support
them.

Release Notes:

- N/A

crates/agent2/src/agent.rs

@@ -913,6 +913,14 @@ impl acp_thread::AgentConnection for NativeAgentConnection {
         })
     }
 
+    fn prompt_capabilities(&self) -> acp::PromptCapabilities {
+        acp::PromptCapabilities {
+            image: true,
+            audio: false,
+            embedded_context: true,
+        }
+    }
+
     fn resume(
         &self,
         session_id: &acp::SessionId,