fix: better readonly checks for proxies

[dummdidumm]

• Expect the thing that's checked to be wrapped with the proxy already, so that we can just check for the state symbol
• Make error message more descriptive

See discussion in #9789 for more info

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🦋 Changeset detected

Latest commit: 3f6a024

The changes in this PR will be included in the next version bump.

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