fix(cloudflare): use correct Proxy receiver in instrumentDurableObjectStorage

[dmmulroy]

Summary

Fixes #19661

instrumentDurableObjectStorage's Proxy get trap passes receiver (the proxy) to Reflect.get, breaking native workerd getters like storage.sql that validate this via internal slots.

• Change Reflect.get(target, prop, receiver) → Reflect.get(target, prop, target) so native getters execute with the real storage object as this
• Add regression tests using a class with private fields to simulate workerd's native brand-checked getters

Details

The sql property on DurableObjectStorage is a native getter that requires the real native object as this. When the Proxy's get trap calls Reflect.get(target, prop, receiver), the getter runs with this = proxy → "Illegal invocation". Using target as receiver ensures native getters always run against the real storage object.

Instrumented KV methods (get, put, delete, list) were unaffected because they're functions that get explicitly .bind(target)ed or called via .apply(target, args). The bug only manifests for non-function getters (like sql).

Regression tests use a BrandCheckedStorage class with private fields — accessing #sqlInstance on the wrong this throws TypeError, faithfully simulating workerd's native internal-slot validation.