fix!: guarantee immutable config updates for plugins

[AlessioGr]

Why

It's easy to introduce hard-to-debug bugs when parts of a config object are shared across multiple locations and plugins mutate these shared references directly without creating shallow copies.. By switching to Immer, every update goes through structural sharing, so each plugin works with an immutable snapshot instead of a live object.

What’s changed

• All config mutations now go through Immer
  • Guarantees immutability and eliminates accidental cross-plugin side effects.

⚠️ BREAKING CHANGE

Plugins must not capture the config argument in long-lived closures (e.g. route or endpoint handlers).
Once plugins have run, Immer revokes the draft proxy; any later access throws a

TypeError: Cannot perform 'get' on a proxy that has been revoked

See the Immer discussion: immerjs/immer#430.

Recommended migration path

Access configuration at run time via req.payload.config instead of the plugin-time config argument. Alternatively, we could pass the non-proxied config as second argument to the plugin function