Auto oss vs cost efficient 50/50 A/B test (#9355)

I want to run an a/b test where 50% of free users get defaulted to auto
cost efficient, and other 50% to auto open weights.

## Tests
- 50% of the time when i open the app with a different WARP_DATA_DIR i
get the expected flip/flopping
- my default gets persisted throughout signup

---------

Co-authored-by: Oz <oz-agent@warp.dev>

app/src/ai/onboarding.rs

@@ -7,10 +7,15 @@ use warp_core::ui::icons::Icon;
 use warpui::{AppContext, SingletonEntity};
 
 use crate::auth::AuthStateProvider;
+use crate::experiments::FreeTierDefaultModel;
 use crate::workspaces::user_workspaces::UserWorkspaces;
 
 use super::llms::{DisableReason, LLMInfo, LLMPreferences};
 
+/// mirrors server-side model ids
+const AUTO_OPEN_LLM_ID: &str = "auto-open";
+const AUTO_COST_EFFICIENT_LLM_ID: &str = "auto-efficient";
+
 impl From<&LLMInfo> for OnboardingModelInfo {
     fn from(llm: &LLMInfo) -> Self {
         Self {
@@ -36,6 +41,27 @@ pub fn build_onboarding_models(prefs: &LLMPreferences) -> (Vec<OnboardingModelIn
     (models, default_id)
 }
 
+pub fn apply_free_tier_default_model_override(
+    models: &mut [OnboardingModelInfo],
+    server_default_id: LLMId,
+    ctx: &mut AppContext,
+) -> LLMId {
+    // server only gives back cost-efficient as a default if you're on a free or no plan
+    // if you ARE on some sort of plan... we should respect what the server says
+    if server_default_id != LLMId::from(AUTO_COST_EFFICIENT_LLM_ID) {
+        return server_default_id;
+    }
+    let auto_open_id = LLMId::from(AUTO_OPEN_LLM_ID);
+    let auto_open_available = models.iter().any(|m| m.id == auto_open_id);
+    if !auto_open_available || !FreeTierDefaultModel::should_default_to_auto_open(ctx) {
+        return server_default_id;
+    }
+    for m in models.iter_mut() {
+        m.is_default = m.id == auto_open_id;
+    }
+    auto_open_id
+}
+
 pub fn current_onboarding_auth_state(ctx: &AppContext) -> OnboardingAuthState {
     let auth_state = AuthStateProvider::as_ref(ctx).get();
     if auth_state.is_anonymous_or_logged_out() {