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/memories/startup/dispatch.rs

@@ -208,7 +208,7 @@ mod tests {
             .await
             .expect("initialize state db");
 
-            let manager = ThreadManager::with_models_provider_and_home(
+            let manager = ThreadManager::with_models_provider_and_home_for_tests(
                 CodexAuth::from_api_key("dummy"),
                 config.model_provider.clone(),
                 config.codex_home.clone(),