protondrive: refuse to write names which aren't valid UTF-8

With an encoding which lets invalid UTF-8 through, rclone could create
a file or folder whose name Proton's clients, and rclone itself, then
refuse to read, so it vanished from listings. Such names are now
refused when writing.

Found by FuzzValidateName.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/protondrive/crypto.go

@@ -505,6 +505,8 @@ func validateName(name string) error {
 		return fmt.Errorf("name must not be %q", name)
 	case strings.ContainsAny(name, "/\x00"):
 		return errors.New("name must not contain / or NUL")
+	case !utf8.ValidString(name):
+		return errors.New("name must be valid UTF-8")
 	}
 	n := 0
 	for _, r := range name {

backend/protondrive/crypto_internal_test.go

@@ -380,6 +380,9 @@ func TestValidateName(t *testing.T) {
 		// these takes two.
 		{strings.Repeat("😀", 127) + "a", true},
 		{strings.Repeat("😀", 128), false},
+		// Other clients couldn't read it back: decryptName refuses it.
+		{"a\xffb", false},
+		{"\xed\xa0\x80", false}, // a UTF-16 surrogate
 	} {
 		err := validateName(test.name)
 		if test.ok {

backend/protondrive/fuzz_internal_test.go

@@ -103,6 +103,7 @@ func FuzzValidateName(f *testing.F) {
 			return
 		}
 		assert.NotEmpty(t, name)
+		assert.True(t, utf8.ValidString(name), "%q", name)
 		assert.NotEqual(t, ".", name)
 		assert.NotEqual(t, "..", name)
 		assert.NotContains(t, name, "/")