Expose model settings in app-server thread metadata (#42151)

## What changed

- Add nullable `model` and `reasoningEffort` fields to the shared app-server `Thread` object and generated schemas.
- Report current settings for loaded threads and the latest persisted settings for unloaded threads across thread read, list, start, resume, rollback, metadata update, and notification paths.
- Keep unavailable legacy or filesystem-only settings nullable, and allow metadata reads without loading a thread or dispatching queued work.

## Testing

- Cover live-setting precedence, persisted settings after restart, null legacy values, and model metadata returned by thread lifecycle operations.

GitOrigin-RevId: 22ebe8a8d1910a820d90a97730d812a00d8c1372

codex-rs/app-server-protocol/schema/json/v2/ThreadStartedNotification.json

@@ -1104,6 +1104,13 @@
           "description": "Identifier for this thread. Codex-generated thread IDs are UUIDv7.",
           "type": "string"
         },
+        "model": {
+          "description": "Current configured model when loaded, otherwise the latest persisted model. Null when unavailable. This is not per-turn execution telemetry.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "modelProvider": {
           "description": "Model provider used for this thread (for example, 'openai').",
           "type": "string"
@@ -1140,6 +1147,17 @@
             "null"
           ]
         },
+        "reasoningEffort": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/ReasoningEffort"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "description": "Current configured reasoning effort when loaded, otherwise the latest persisted effort. Null when unset or unavailable. This is not per-turn execution telemetry."
+        },
         "recencyAt": {
           "description": "Unix timestamp (in seconds) used for thread recency ordering.",
           "format": "int64",