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/remote_models.rs

@@ -100,9 +100,9 @@ async fn remote_models_get_model_info_uses_longest_matching_prefix() -> Result<(
         base_url: Some(format!("{}/v1", server.uri())),
         ..built_in_model_providers()["openai"].clone()
     };
-    let manager = ModelsManager::with_provider(
+    let manager = codex_core::test_support::models_manager_with_provider(
         codex_home.path().to_path_buf(),
-        codex_core::auth::AuthManager::from_auth_for_testing(auth),
+        codex_core::test_support::auth_manager_from_auth(auth),
         provider,
     );
 
@@ -497,9 +497,9 @@ async fn remote_models_preserve_builtin_presets() -> Result<()> {
         base_url: Some(format!("{}/v1", server.uri())),
         ..built_in_model_providers()["openai"].clone()
     };
-    let manager = ModelsManager::with_provider(
+    let manager = codex_core::test_support::models_manager_with_provider(
         codex_home.path().to_path_buf(),
-        codex_core::auth::AuthManager::from_auth_for_testing(auth),
+        codex_core::test_support::auth_manager_from_auth(auth),
         provider,
     );
 
@@ -562,9 +562,9 @@ async fn remote_models_merge_adds_new_high_priority_first() -> Result<()> {
         base_url: Some(format!("{}/v1", server.uri())),
         ..built_in_model_providers()["openai"].clone()
     };
-    let manager = ModelsManager::with_provider(
+    let manager = codex_core::test_support::models_manager_with_provider(
         codex_home.path().to_path_buf(),
-        codex_core::auth::AuthManager::from_auth_for_testing(auth),
+        codex_core::test_support::auth_manager_from_auth(auth),
         provider,
     );
 
@@ -613,9 +613,9 @@ async fn remote_models_merge_replaces_overlapping_model() -> Result<()> {
         base_url: Some(format!("{}/v1", server.uri())),
         ..built_in_model_providers()["openai"].clone()
     };
-    let manager = ModelsManager::with_provider(
+    let manager = codex_core::test_support::models_manager_with_provider(
         codex_home.path().to_path_buf(),
-        codex_core::auth::AuthManager::from_auth_for_testing(auth),
+        codex_core::test_support::auth_manager_from_auth(auth),
         provider,
     );
 
@@ -661,9 +661,9 @@ async fn remote_models_merge_preserves_bundled_models_on_empty_response() -> Res
         base_url: Some(format!("{}/v1", server.uri())),
         ..built_in_model_providers()["openai"].clone()
     };
-    let manager = ModelsManager::with_provider(
+    let manager = codex_core::test_support::models_manager_with_provider(
         codex_home.path().to_path_buf(),
-        codex_core::auth::AuthManager::from_auth_for_testing(auth),
+        codex_core::test_support::auth_manager_from_auth(auth),
         provider,
     );
 
@@ -706,9 +706,9 @@ async fn remote_models_request_times_out_after_5s() -> Result<()> {
         base_url: Some(format!("{}/v1", server.uri())),
         ..built_in_model_providers()["openai"].clone()
     };
-    let manager = ModelsManager::with_provider(
+    let manager = codex_core::test_support::models_manager_with_provider(
         codex_home.path().to_path_buf(),
-        codex_core::auth::AuthManager::from_auth_for_testing(auth),
+        codex_core::test_support::auth_manager_from_auth(auth),
         provider,
     );
 
@@ -773,9 +773,9 @@ async fn remote_models_hide_picker_only_models() -> Result<()> {
         base_url: Some(format!("{}/v1", server.uri())),
         ..built_in_model_providers()["openai"].clone()
     };
-    let manager = ModelsManager::with_provider(
+    let manager = codex_core::test_support::models_manager_with_provider(
         codex_home.path().to_path_buf(),
-        codex_core::auth::AuthManager::from_auth_for_testing(auth),
+        codex_core::test_support::auth_manager_from_auth(auth),
         provider,
     );