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/suite/v2/collaboration_mode_list.rs

@@ -15,7 +15,7 @@ use codex_app_server_protocol::CollaborationModeListParams;
 use codex_app_server_protocol::CollaborationModeListResponse;
 use codex_app_server_protocol::JSONRPCResponse;
 use codex_app_server_protocol::RequestId;
-use codex_core::models_manager::test_builtin_collaboration_mode_presets;
+use codex_core::test_support::builtin_collaboration_mode_presets;
 use codex_protocol::config_types::CollaborationModeMask;
 use codex_protocol::config_types::ModeKind;
 use pretty_assertions::assert_eq;
@@ -55,7 +55,7 @@ async fn list_collaboration_modes_returns_presets() -> Result<()> {
 /// If the defaults change in the app server, this helper should be updated alongside the
 /// contract, or the test will fail in ways that imply a regression in the API.
 fn plan_preset() -> CollaborationModeMask {
-    let presets = test_builtin_collaboration_mode_presets();
+    let presets = builtin_collaboration_mode_presets();
     presets
         .into_iter()
         .find(|p| p.mode == Some(ModeKind::Plan))
@@ -64,7 +64,7 @@ fn plan_preset() -> CollaborationModeMask {
 
 /// Builds the default preset that the list response is expected to return.
 fn default_preset() -> CollaborationModeMask {
-    let presets = test_builtin_collaboration_mode_presets();
+    let presets = builtin_collaboration_mode_presets();
     presets
         .into_iter()
         .find(|p| p.mode == Some(ModeKind::Default))