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/lib.rs

@@ -20,7 +20,8 @@ pub mod test_codex_exec;
 
 #[ctor]
 fn enable_deterministic_unified_exec_process_ids_for_tests() {
-    codex_core::set_deterministic_process_ids_for_tests(true);
+    codex_core::test_support::set_thread_manager_test_mode(true);
+    codex_core::test_support::set_deterministic_process_ids(true);
 }
 
 #[track_caller]