fix(storage): preserve ReadParts errors across keepalive responses

ReadPartsHandler completed its keepalive stream before reporting storage failures, then tried to write an ordinary error response after the body was already owned. Clients consequently decoded the error text as msgpack and lost the real failure.

Send failures through the keepalive completion channel and mark success only after ReadParts returns cleanly, preserving the existing wire framing and error identity.

Co-authored-by: ChatGPT <noreply@openai.com>
Co-authored-by: Claude <noreply@anthropic.com>

cmd/storage-rest-server-guard_test.go

@@ -190,13 +190,22 @@ func TestIsVolumeRootAlias(t *testing.T) {
 		want bool
 	}{
 		// Collapse back to the volume directory.
-		{"", true}, {"/", true}, {"//", true},
+		{"", true},
+		{"/", true},
+		{"//", true},
 		// Backslash is platform-dependent; see TestIsVolumeRootAliasIsPlatformCorrect.
 		// Whitespace is NOT a separator. path.Clean leaves it alone, so these
 		// name real directories and are legal S3 object keys.
-		{" ", false}, {"  ", false}, {"\t", false}, {"\n", false},
-		{" / ", false}, {"/ ", false},
-		{"a", false}, {"/a", false}, {"..", false}, {" a ", false},
+		{" ", false},
+		{"  ", false},
+		{"\t", false},
+		{"\n", false},
+		{" / ", false},
+		{"/ ", false},
+		{"a", false},
+		{"/a", false},
+		{"..", false},
+		{" a ", false},
 		{"obj/part.1", false},
 	} {
 		if got := isVolumeRootAlias(tc.path); got != tc.want {
@@ -239,12 +248,19 @@ func TestIsVolumeRootAliasIsPlatformCorrect(t *testing.T) {
 		// Space and period: ordinary filename characters on Unix, but stripped
 		// from a component by Win32 normalisation, so a component made only of
 		// them vanishes and the path resolves to the volume root.
-		{" ", false, true}, {"  ", false, true}, {" / ", false, true},
-		{"...", false, true}, {". .", false, true},
+		{" ", false, true},
+		{"  ", false, true},
+		{" / ", false, true},
+		{"...", false, true},
+		{". .", false, true},
 		// Never an alias anywhere.
-		{"\t", false, false}, {"\n", false, false},
-		{"a", false, false}, {"/a", false, false}, {"a ", false, false},
-		{" a", false, false}, {"a.", false, false},
+		{"\t", false, false},
+		{"\n", false, false},
+		{"a", false, false},
+		{"/a", false, false},
+		{"a ", false, false},
+		{" a", false, false},
+		{"a.", false, false},
 	} {
 		if got := isVolumeRootAliasOn(tc.path, false); got != tc.unix {
 			t.Errorf("isVolumeRootAliasOn(%q, unix) = %v, want %v", tc.path, got, tc.unix)