fix(ui): prevent infinite re-render loop on agent configure page

MarkdownEditor's plugins useMemo depended on imageUploadHandler, which
was a new arrow function on every parent render. This caused MDXEditor
to reinitialize plugins on each render, firing onChange, which updated
the overlay state in AgentConfigForm, triggering a parent re-render —
creating an infinite "Maximum update depth exceeded" loop.

Fix: use a stable ref for imageUploadHandler so plugins are only created
once (keyed on whether a handler exists, not its identity). Also use a
module-level empty object for env config fallback to avoid unnecessary
EnvVarEditor re-renders.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/components/AgentConfigForm.tsx

@@ -85,6 +85,9 @@ const emptyOverlay: Overlay = {
   runtime: {},
 };
 
+/** Stable empty object used as fallback for missing env config to avoid new-object-per-render. */
+const EMPTY_ENV: Record<string, EnvBinding> = {};
+
 function isOverlayDirty(o: Overlay): boolean {
   return (
     Object.keys(o.identity).length > 0 ||
@@ -617,8 +620,8 @@ export function AgentConfigForm(props: AgentConfigFormProps) {
                 <EnvVarEditor
                   value={
                     isCreate
-                      ? ((val!.envBindings ?? {}) as Record<string, EnvBinding>)
-                      : ((eff("adapterConfig", "env", config.env ?? {}) as Record<string, EnvBinding>)
+                      ? ((val!.envBindings ?? EMPTY_ENV) as Record<string, EnvBinding>)
+                      : ((eff("adapterConfig", "env", (config.env ?? EMPTY_ENV) as Record<string, EnvBinding>))
                       )
                   }
                   secrets={availableSecrets}