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/core/config.schema.json

@@ -1594,6 +1594,14 @@
       "format": "int64",
       "type": "integer"
     },
+    "model_catalog_json": {
+      "allOf": [
+        {
+          "$ref": "#/definitions/AbsolutePathBuf"
+        }
+      ],
+      "description": "Optional path to a JSON file containing a complete model catalog. When set, this replaces the bundled catalog for this process."
+    },
     "model_context_window": {
       "description": "Size of the context window for the model, in tokens.",
       "format": "int64",