Cloudflare Agent RPC proxy breaks native private fields

[sergical]

SDK

@sentry/cloudflare 10.69.0 with Cloudflare Agents 0.20.1.

Reproduction

1. Wrap a supervisor Agent with instrumentAgentWithSentry and set enableRpcTracePropagation: true.
2. Address either a top-level Agent with getAgentByName() or a child with native Durable Object facets (ctx.facets.get()), as Agents SDK subAgent() does.
3. Let the Agents SDK bootstrap the instance through native Durable Object RPC (setName()).

Facets and direct RPC on their Durable Object-style stubs are documented Cloudflare runtime behavior: https://developers.cloudflare.com/dynamic-workers/usage/durable-object-facets/

Expected

Native Agent/facet/DO RPC invokes the method with the underlying Durable Object instance as its receiver.

Actual

Sentry returns a proxy from finalizeWithRpcInstrumentation, and native RPC invokes the prototype method with that proxy as this. Any native private field access then fails. PartyServer's setName() exposes this concretely through its #_name field:

TypeError: Cannot read private member #_name from an object whose class did not declare it

Setting enableRpcTracePropagation: false avoids the error.

[linear-code]

JS-3283

[JPeer264]

I'm on it. It is just odd as this scenario should have tests as it came up before already. I'll investigate why the tests were not good enough or if anything was missing