## 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`