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/core/tests/suite/resume_warning.rs

@@ -1,9 +1,7 @@
 #![allow(clippy::unwrap_used, clippy::expect_used)]
 
-use codex_core::AuthManager;
 use codex_core::CodexAuth;
 use codex_core::NewThread;
-use codex_core::ThreadManager;
 use codex_core::protocol::EventMsg;
 use codex_core::protocol::InitialHistory;
 use codex_core::protocol::ResumedHistory;
@@ -58,11 +56,12 @@ async fn emits_warning_when_resumed_model_differs() {
 
     let initial_history = resume_history(&config, "previous-model", &rollout_path);
 
-    let thread_manager = ThreadManager::with_models_provider(
+    let thread_manager = codex_core::test_support::thread_manager_with_models_provider(
         CodexAuth::from_api_key("test"),
         config.model_provider.clone(),
     );
-    let auth_manager = AuthManager::from_auth_for_testing(CodexAuth::from_api_key("test"));
+    let auth_manager =
+        codex_core::test_support::auth_manager_from_auth(CodexAuth::from_api_key("test"));
 
     // Act: resume the conversation.
     let NewThread {