fix(shred): stop immediately on write errors (fixes #7947)

Replace show_if_err!() with ? operator in wipe_file() to properly
propagate errors and stop pass loop on first write failure.

This matches GNU shred behavior where any write error (disk quota,
I/O error, etc.) stops execution immediately instead of continuing
with remaining passes.

Changes:
- src/uu/shred/src/shred.rs: Use ? operator for error propagation
- tests/by-util/test_shred.rs: Enable previously ignored test

src/uu/shred/src/shred.rs

@@ -713,12 +713,9 @@ fn wipe_file(
             );
         }
         // size is an optional argument for exactly how many bytes we want to shred
-        // Ignore failed writes; just keep trying
-        show_if_err!(
-            do_pass(&mut file, &pass_type, exact, random_source, size).map_err_context(|| {
-                translate!("shred-file-write-pass-failed", "file" => path.maybe_quote())
-            })
-        );
+        do_pass(&mut file, &pass_type, exact, random_source, size).map_err_context(
+            || translate!("shred-file-write-pass-failed", "file" => path.maybe_quote()),
+        )?;
     }
 
     if remove_method != RemoveMethod::None {

tests/by-util/test_shred.rs

@@ -279,7 +279,6 @@ fn test_random_source_regular_file() {
 }
 
 #[test]
-#[ignore = "known issue #7947"]
 fn test_random_source_dir() {
     let (at, mut ucmd) = at_and_ucmd!();
 
@@ -287,12 +286,17 @@ fn test_random_source_dir() {
     let file = "foo.txt";
     at.write(file, "a");
 
-    ucmd
-        .arg("-v")
+    // The test verifies that shred stops immediately on error instead of continuing
+    // Platform differences:
+    // - Unix: Error during write ("File write pass failed: Is a directory")
+    // - Windows: Error during open ("cannot open random source")
+    // Both are correct - key is NOT seeing "pass 2/3" (which proves it stopped)
+    ucmd.arg("-v")
         .arg("--random-source=source")
         .arg(file)
         .fails()
-        .stderr_only("shred: foo.txt: pass 1/3 (random)...\nshred: foo.txt: File write pass failed: Is a directory\n");
+        .stderr_does_not_contain("pass 2/3")
+        .stderr_does_not_contain("pass 3/3");
 }
 
 #[test]