Add sha256 integrity to "com_google_protobuf_v25.0" which was in WORKSPACE but missing from WORKSPACE.bzlmod

This should silence the DEBUG warning
```
DEBUG: Rule 'com_google_protobuf_v25.0' indicated that a canonical reproducible form can be obtained by modifying arguments integrity = "sha256-e+7ZxRHWMs/3wirACU3Xcg5VAVMDnV2n4Fm8zrSIR0o="
```
PiperOrigin-RevId: 714142857

WORKSPACE.bzlmod

@@ -26,6 +26,7 @@ register_toolchains("@system_ruby//:toolchain")
 # For testing runtime against old gencode from a previous major version.
 http_archive(
     name = "com_google_protobuf_v25.0",
+    integrity = "sha256-e+7ZxRHWMs/3wirACU3Xcg5VAVMDnV2n4Fm8zrSIR0o=",
     strip_prefix = "protobuf-25.0",
     url = "https://github.com/protocolbuffers/protobuf/releases/download/v25.0/protobuf-25.0.tar.gz",
 )