Replace Zod schema's internal _def.value access with a direct string
literal. The previous approach failed due to multiple Zod instances
being bundled, causing schema.shape.method._def.value to return
undefined.
The fix uses a typed string constant that provides compile-time safety
via McpUiSandboxProxyReadyNotification["method"] while avoiding runtime
dependency on Zod internals.
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Co-authored-by: Claude <noreply@anthropic.com>