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/experiments/mod.rs

@@ -9,6 +9,7 @@ mod block_onboarding_layer;
 mod login_layer;
 mod rendering;
 pub use block_onboarding_layer::{BlockOnboarding, BLOCK_ONBOARDING_LAYER};
+pub use free_tier_default_model_layer::{FreeTierDefaultModel, FREE_TIER_DEFAULT_MODEL_LAYER};
 pub use improved_palette_search_layer::{ImprovedPaletteSearch, IMPROVED_PALETTE_SEARCH_LAYER};
 pub use login_layer::{AuthFlowInstructions, LOGIN_LAYER};
 use warp_core::user_preferences::GetUserPreferences as _;
@@ -69,6 +70,7 @@ lazy_static! {
         &*BLOCK_ONBOARDING_LAYER,
         &*rendering::LAYER,
         &*IMPROVED_PALETTE_SEARCH_LAYER,
+        &*FREE_TIER_DEFAULT_MODEL_LAYER,
     ];
 
     /// Mapping of experiments to their respective layers. The mappings are built up
@@ -403,6 +405,7 @@ pub fn init(ctx: &mut AppContext) {
 #[path = "mod_tests.rs"]
 mod tests;
 
+mod free_tier_default_model_layer;
 mod improved_palette_search_layer;
 #[cfg(test)]
 mod validation_tests;