client side modelinfo overrides (#12101)

TL;DR
Add top-level `model_catalog_json` config support so users can supply a
local model catalog override from a JSON file path (including adding new
models) without backend changes.

### Problem
Codex previously had no clean client-side way to replace/overlay model
catalog data for local testing of model metadata and new model entries.

### Fix
- Add top-level `model_catalog_json` config field (JSON file path).
- Apply catalog entries when resolving `ModelInfo`:
  1. Base resolved model metadata (remote/fallback)
  2. Catalog overlay from `model_catalog_json`
3. Existing global top-level overrides (`model_context_window`,
`model_supports_reasoning_summaries`, etc.)

### Note
Will revisit per-field overrides in a follow-up

### Tests
Added tests

codex-rs/exec/src/lib.rs

@@ -373,6 +373,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         config.codex_home.clone(),
         auth_manager.clone(),
         SessionSource::Exec,
+        config.model_catalog.clone(),
     ));
     let default_model = thread_manager
         .get_models_manager()