Single message finalizing appendable upload returns incorrect metadata

Testbench v0.55.0

Testbench does not currently return correct metadata for a single message that should create and finalize the upload.

### Request
```
BidiWriteObjectRequest{
write_object_spec {
resource { name: "my-object" bucket: "projects/_/buckets/my-bucket" }
appendable: true
}
checksummed_data { content: "ABDE" crc32c: 1939456564 }
object_checksums { crc32c: 1939456564 }
finish_write: true
}
```

### Response
```
BidiWriteObjectResponse{
resource {
name: "my-object"
bucket: "my-bucket"
generation: 1753465277702
metageneration: 1
create_time { seconds: 1753467592 nanos: 932413000 }
checksums { crc32c: 1939456564 md5_hash: "\324\035\214\331\217\000\262\004\351\200\t\230\354\370B~" }
update_time { seconds: 1753467592 nanos: 932413000 }
metadata { key: "x_emulator_crc32c" value: "c5nCNA==" }
metadata { key: "x_emulator_no_md5" value: "true" }
metadata { key: "x_emulator_upload" value: "grpc" }
owner { entity: "user-object.owners@example.com" }
etag: "c4ca4238a0b923820dcc509a6f75849b"
finalize_time { seconds: 1753467592 nanos: 932587000 }
}
write_handle { handle: "ef6892351f0affef044b62f4712b05af88b45245e6f7bd44b62d9e873022bf05" }
}

OK
```