test: fix HTTP 500 error pattern to match actual error message

Update test expectation for server error to match the error format
from downloadWithLock() which throws "Download failed: HTTP 500
Internal Server Error" instead of "Failed to download binary".

Author: jdalton

test/dlx-binary.test.ts

@@ -264,7 +264,7 @@ describe.sequential('dlx-binary', () => {
               name: 'error-binary',
               url,
             }),
-          ).rejects.toThrow(/Failed to download binary/)
+          ).rejects.toThrow(/Download failed: HTTP 500/)
         } finally {
           restoreHome()
         }