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/common/test_codex.rs

@@ -170,7 +170,7 @@ impl TestCodexBuilder {
         resume_from: Option<PathBuf>,
     ) -> anyhow::Result<TestCodex> {
         let auth = self.auth.clone();
-        let thread_manager = ThreadManager::with_models_provider_and_home(
+        let thread_manager = codex_core::test_support::thread_manager_with_models_provider_and_home(
             auth.clone(),
             config.model_provider.clone(),
             config.codex_home.clone(),
@@ -179,7 +179,7 @@ impl TestCodexBuilder {
 
         let new_conversation = match resume_from {
             Some(path) => {
-                let auth_manager = codex_core::AuthManager::from_auth_for_testing(auth);
+                let auth_manager = codex_core::test_support::auth_manager_from_auth(auth);
                 thread_manager
                     .resume_thread_from_rollout(config.clone(), path, auth_manager)
                     .await?