Remove TUI legacy core test_support dependencies (#27484)

## Why

The TUI now sits on the app-server layer, but
`app-server-client::legacy_core` still exposed core test helpers solely
for TUI tests. We've been whittling away the remaining dependencies.
This is the next step on that journey.

There is no functional change — just a refactor, and this affects only
test code, so it should be low risk.

## What changed

- remove the `legacy_core::test_support` re-export and call
model-manager test helpers directly
- keep the bundled model-preset cache local to TUI test support
- import constraint types directly from `codex-config`

codex-rs/app-server-client/src/lib.rs

@@ -90,10 +90,6 @@ pub mod legacy_core {
         pub use codex_core::personality_migration::*;
     }
 
-    pub mod test_support {
-        pub use codex_core::test_support::*;
-    }
-
     pub mod windows_sandbox {
         pub use codex_core::windows_sandbox::*;
     }