Add oauth_resource handling for MCP login flows (#12866)

Addresses bug https://github.com/openai/codex/issues/12589

Builds on community PR #12763.

This adds `oauth_resource` support for MCP `streamable_http` servers and
wires it through the relevant config and login paths. It fixes the bug
where the configured OAuth resource was not reliably included in the
authorization request, causing MCP login to omit the expected
`resource` parameter.

codex-rs/core/config.schema.json

@@ -1146,6 +1146,10 @@
           },
           "type": "object"
         },
+        "oauth_resource": {
+          "default": null,
+          "type": "string"
+        },
         "required": {
           "default": null,
           "type": "boolean"