Studio: pre-check Codex default models when creating the connection

First-run UX bug surfaced by an end-to-end probe: after the user adds
the "OpenAI Codex (local CLI)" connection from Settings -> Connections
-> Add provider, every model in the form starts UNCHECKED. The user
saves, returns to the chat composer, opens the model picker -- and
the "Connected" tab is missing because no Codex models are enabled.
The user has to re-open the connection, tick at least one model, save
again, then return to chat. Two round-trips to make a feature work
that the rest of the UI already advertises as installed.

Anthropic / OpenAI / OpenRouter all keep their explicit-opt-in
defaults because the model choice has billing and capability impact.
Codex is the local CLI on the same machine -- the SDK accepts any
model id, the "default_models" list is the SDK's curated shortlist,
and gating chat behind a manual click adds friction without buying
anything. Pre-checking all default models for Codex (and only Codex)
makes the path "click Add -> click Save -> chat works" survive a
single click sequence, matching the empirical setup we walked
through in the e2e probe.

studio/frontend/src/features/chat/chat-providers-dialog.tsx

@@ -365,8 +365,18 @@ export function ChatProvidersSettings({
     // providers and local OpenAI-compat presets stay empty until the user
     // clicks "Load available models".
     const seedDefaults = entry.model_list_mode === "curated";
-    setAvailableModels(seedDefaults ? [...entry.default_models] : []);
-    setSelectedModelIds([]);
+    const defaults = seedDefaults ? [...entry.default_models] : [];
+    setAvailableModels(defaults);
+    // Codex is a local CLI, not a metered cloud account, so checking all of
+    // the SDK's default model ids by default is safe and avoids the
+    // first-run UX trap where users create the connection, never check any
+    // model, and then the "Connected" tab silently never appears in the
+    // chat model picker. Anthropic / OpenAI / etc. still need explicit
+    // model selection because the choice has billing and capability
+    // consequences.
+    setSelectedModelIds(
+      providerType === CODEX_PROVIDER_TYPE ? defaults : [],
+    );
     setManualModelIds("");
     setModelSearchQuery("");
     setBaseUrlDraft("");