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/unified_exec/mod.rs

@@ -42,8 +42,7 @@ mod head_tail_buffer;
 mod process;
 mod process_manager;
 
-#[cfg(any(test, feature = "test-support"))]
-pub fn set_deterministic_process_ids_for_tests(enabled: bool) {
+pub(crate) fn set_deterministic_process_ids_for_tests(enabled: bool) {
     process_manager::set_deterministic_process_ids_for_tests(enabled);
 }