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/src/test_support.rs

@@ -0,0 +1,78 @@
+//! Test-only helpers exposed for cross-crate integration tests.
+//!
+//! Production code should not depend on this module.
+//! We prefer this to using a crate feature to avoid building multiple
+//! permutations of the crate.
+
+use std::path::PathBuf;
+use std::sync::Arc;
+
+use codex_protocol::config_types::CollaborationModeMask;
+use codex_protocol::openai_models::ModelInfo;
+use codex_protocol::openai_models::ModelPreset;
+
+use crate::AuthManager;
+use crate::CodexAuth;
+use crate::ModelProviderInfo;
+use crate::ThreadManager;
+use crate::config::Config;
+use crate::models_manager::collaboration_mode_presets;
+use crate::models_manager::manager::ModelsManager;
+use crate::models_manager::model_presets;
+use crate::thread_manager;
+use crate::unified_exec;
+
+pub fn set_thread_manager_test_mode(enabled: bool) {
+    thread_manager::set_thread_manager_test_mode_for_tests(enabled);
+}
+
+pub fn set_deterministic_process_ids(enabled: bool) {
+    unified_exec::set_deterministic_process_ids_for_tests(enabled);
+}
+
+pub fn auth_manager_from_auth(auth: CodexAuth) -> Arc<AuthManager> {
+    AuthManager::from_auth_for_testing(auth)
+}
+
+pub fn auth_manager_from_auth_with_home(auth: CodexAuth, codex_home: PathBuf) -> Arc<AuthManager> {
+    AuthManager::from_auth_for_testing_with_home(auth, codex_home)
+}
+
+pub fn thread_manager_with_models_provider(
+    auth: CodexAuth,
+    provider: ModelProviderInfo,
+) -> ThreadManager {
+    ThreadManager::with_models_provider_for_tests(auth, provider)
+}
+
+pub fn thread_manager_with_models_provider_and_home(
+    auth: CodexAuth,
+    provider: ModelProviderInfo,
+    codex_home: PathBuf,
+) -> ThreadManager {
+    ThreadManager::with_models_provider_and_home_for_tests(auth, provider, codex_home)
+}
+
+pub fn models_manager_with_provider(
+    codex_home: PathBuf,
+    auth_manager: Arc<AuthManager>,
+    provider: ModelProviderInfo,
+) -> ModelsManager {
+    ModelsManager::with_provider_for_tests(codex_home, auth_manager, provider)
+}
+
+pub fn get_model_offline(model: Option<&str>) -> String {
+    ModelsManager::get_model_offline_for_tests(model)
+}
+
+pub fn construct_model_info_offline(model: &str, config: &Config) -> ModelInfo {
+    ModelsManager::construct_model_info_offline_for_tests(model, config)
+}
+
+pub fn all_model_presets() -> &'static Vec<ModelPreset> {
+    &model_presets::PRESETS
+}
+
+pub fn builtin_collaboration_mode_presets() -> Vec<CollaborationModeMask> {
+    collaboration_mode_presets::builtin_collaboration_mode_presets()
+}