fix(kuna): retain fallback diagnostics for empty stderr

Refs #4374.

Preserve the command error when stderr contains no diagnostic text.

src/kuna/KunaCommand.ts

@@ -346,7 +346,7 @@ const commandFailure = (
     kind: "failed",
     error: new ProviderAdapterError(KUNA_PROVIDER_ID, operation, {
       ...details,
-      userMessage: `kuna ${subcommand} failed${exitCode === undefined || exitCode === null ? "" : ` with exit code ${String(exitCode)}`}: ${capturedOutput?.stderr.trim().split("\n").at(-1) ?? cause.message}`,
+      userMessage: `kuna ${subcommand} failed${exitCode === undefined || exitCode === null ? "" : ` with exit code ${String(exitCode)}`}: ${capturedOutput?.stderr.trim().split("\n").at(-1) || cause.message}`,
     }),
   };
 };