fix(test): skip TestLocalResolver_BubblesUpCopyFailure when uid==0

os.Chmod(dst, 0o555) silently passes when os.Geteuid() == 0 because
root bypasses POSIX permission checks. A previous attempt to use a
symlink to /dev/full also fails: Go's os.MkdirAll resolves the symlink
during path traversal and the kernel allows mkdir("/dev/full") as a
device-table entry — io.Copy to /dev/full then succeeds with 0 bytes
written and returns nil.

The honest, consistent fix mirrors TestLocalResolver_CopyFileSourceUnreadable:
skip when running as root. The write-failure propagation logic is
exercised correctly in non-root CI environments.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace-server/internal/plugins/local_test.go

@@ -132,14 +132,19 @@ func TestLocalResolver_HonoursContextCancellation(t *testing.T) {
 }
 
 func TestLocalResolver_BubblesUpCopyFailure(t *testing.T) {
-	// Source file the copyTree walk would read; make dst unwritable so
-	// the copyFile step fails.
+	// os.Chmod(dst, 0o555) silently passes when os.Geteuid() == 0
+	// (root bypasses POSIX permission checks).  We cannot reliably
+	// exercise the write-failure branch in a root environment without
+	// patching the syscalls, so skip it honestly.
+	if os.Getuid() == 0 {
+		t.Skip("running as root — cannot exercise write-failure branch")
+	}
+
 	base := t.TempDir()
 	writePlugin(t, base, "demo", map[string]string{
 		"plugin.yaml": "name: demo\n",
 	})
 	dst := t.TempDir()
-	// Make dst read-only so creating files inside it fails.
 	if err := os.Chmod(dst, 0o555); err != nil {
 		t.Fatal(err)
 	}