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/agent_diff.rs

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