Prepare MCP connections for coordinated OAuth refresh (#42128)

## What changed

- Add the under-development `mcp_oauth_refresh_coordination` feature and pin its selected refresh mode to each MCP connection.
- Treat the refresh mode as part of OAuth connection identity so runtime and MCP configuration refreshes reconnect when the mode changes.
- Keep legacy refresh behavior in builds where coordination is unavailable and emit a warning when coordinated mode is selected.

## Testing

- Cover connection replacement when the feature is toggled through both runtime and MCP configuration refresh paths.

GitOrigin-RevId: 89596150f0338e6ede0e712badecea4450831766

codex-rs/core/config.schema.json

@@ -822,6 +822,9 @@
             "mcp_2026_07_28": {
               "type": "boolean"
             },
+            "mcp_oauth_refresh_coordination": {
+              "type": "boolean"
+            },
             "memories": {
               "type": "boolean"
             },
@@ -6111,6 +6114,9 @@
         "mcp_2026_07_28": {
           "type": "boolean"
         },
+        "mcp_oauth_refresh_coordination": {
+          "type": "boolean"
+        },
         "memories": {
           "type": "boolean"
         },