Reserve an failure detail code in `Filesystem` category.

PiperOrigin-RevId: 347691385

src/main/protobuf/failure_details.proto

diff --git a/src/main/protobuf/failure_details.proto b/src/main/protobuf/failure_details.proto
index 83110b47..1c31f87 100644
--- a/src/main/protobuf/failure_details.proto
+++ b/src/main/protobuf/failure_details.proto
@@ -470,6 +470,8 @@
     SERVER_FILE_WRITE_FAILURE = 5 [(metadata) = { exit_code: 1 }];
     DEFAULT_DIGEST_HASH_FUNCTION_INVALID_VALUE = 6
         [(metadata) = { exit_code: 2 }];
+
+    reserved 7; // For internal use
   }
 
   Code code = 1;