Fixes #455: unhandled method returns an error, unblocking client

[0x6362]

Fixes #417, #455 - pulls the code from #386 / #393 into a separate patch to handle proliferation of custom ACP message types.

Checklist

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[xenodium]

patch to handle proliferation of custom ACP

Thank you for the PR! This helps surface agents sending non-spec ACP but also enables them to continue working.