fix(ui): wire allowEmpty for embedding provider select (#112)

The `allowEmpty` prop on `ProviderModelSelect` was defined but the
Select dropdown still auto-selected the first provider when the value
was empty, and there was no way to reset back to empty (auto-detect).

- Map empty provider to a sentinel so Radix Select can represent it
- Show an "(auto)" option in the dropdown when `allowEmpty` is set
- Pass `allowEmpty` from both embedding config callers
- Skip clearing the model on provider change in allowEmpty mode to
  avoid a stale-closure bug where the second setState overwrites the
  provider update

Co-authored-by: Luvu182 <208665161+Luvu182@users.noreply.github.com>

ui/web/src/components/shared/provider-model-select.tsx

@@ -97,7 +97,13 @@ export function ProviderModelSelect({
 
   const handleProviderChange = (v: string) => {
     onProviderChange(v);
-    onModelChange("");
+    // Only clear model when NOT in allowEmpty mode.
+    // In allowEmpty mode (embedding config), both callbacks update the same
+    // parent state object — calling onModelChange("") with a stale closure
+    // would overwrite the provider change we just made.
+    if (!allowEmpty) {
+      onModelChange("");
+    }
   };
 
   const handleVerify = async () => {
@@ -110,11 +116,14 @@ export function ProviderModelSelect({
       <div className="grid gap-1.5">
         <InfoLabel tip={providerTip ?? t("providerTip")}>{providerLabel ?? t("provider")}</InfoLabel>
         {enabledProviders.length > 0 ? (
-          <Select value={provider} onValueChange={handleProviderChange}>
+          <Select value={provider || "__empty__"} onValueChange={(v) => handleProviderChange(v === "__empty__" ? "" : v)}>
             <SelectTrigger>
               <SelectValue placeholder={providerPlaceholder ?? t("selectProvider")} />
             </SelectTrigger>
             <SelectContent>
+              {allowEmpty && (
+                <SelectItem value="__empty__">{providerPlaceholder || "(auto)"}</SelectItem>
+              )}
               {enabledProviders.map((p) => (
                 <SelectItem key={p.name} value={p.name}>
                   {p.display_name || p.name}

ui/web/src/pages/agents/agent-detail/config-sections/memory-section.tsx

@@ -40,6 +40,7 @@ export function MemorySection({ enabled, value, onToggle, onChange }: MemorySect
         modelTip="Embedding model name (e.g. text-embedding-3-small). Must be supported by the provider."
         providerPlaceholder="(auto)"
         modelPlaceholder="text-embedding-3-small"
+        allowEmpty
       />
       <div className="grid grid-cols-1 gap-4 sm:grid-cols-2">
         <div className="space-y-2">

ui/web/src/pages/config/sections/ai-defaults-section.tsx

@@ -183,6 +183,7 @@ export function AiDefaultsSection({ data, onSave, saving }: Props) {
             modelTip={t("agents.memory.embeddingModelTip")}
             providerPlaceholder="(auto)"
             modelPlaceholder="text-embedding-3-small"
+            allowEmpty
           />
           <div className="grid grid-cols-2 gap-4">
             <Field label={t("agents.memory.maxResults")} tip={t("agents.memory.maxResultsTip")} type="number" value={memory.max_results} onChange={(v) => updateNested("memory", { max_results: Number(v) })} placeholder="6" />