Remove test-support feature from codex-core and replace it with explicit test toggles (#11405)

## Why

`codex-core` was being built in multiple feature-resolved permutations
because test-only behavior was modeled as crate features. For a large
crate, those permutations increase compile cost and reduce cache reuse.

## Net Change

- Removed the `test-support` crate feature and related feature wiring so
`codex-core` no longer needs separate feature shapes for test consumers.
- Standardized cross-crate test-only access behind
`codex_core::test_support`.
- External test code now imports helpers from
`codex_core::test_support`.
- Underlying implementation hooks are kept internal (`pub(crate)`)
instead of broadly public.

## Outcome

- Fewer `codex-core` build permutations.
- Better incremental cache reuse across test targets.
- No intended production behavior change.

codex-rs/app-server/tests/common/models_cache.rs

@@ -1,6 +1,6 @@
 use chrono::DateTime;
 use chrono::Utc;
-use codex_core::models_manager::model_presets::all_model_presets;
+use codex_core::test_support::all_model_presets;
 use codex_protocol::openai_models::ConfigShellToolType;
 use codex_protocol::openai_models::ModelInfo;
 use codex_protocol::openai_models::ModelPreset;