feat(tvix/store): validate blob size in NARRenderer

Make sure the blob size in the current proto node matches what we get
back from the blob backend.

Change-Id: I939fa18f37c7bc86ada8a495c7be622e69ec47f8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8129
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/store/src/nar/mod.rs

@@ -17,6 +17,9 @@ pub enum RenderError {
     #[error("unable to find blob {}, referred from {}", BASE64.encode(.0), .1)]
     BlobNotFound(Vec<u8>, String),
 
+    #[error("unexpected size in metadata for blob {}, referred from {} returned, expected {}, got {}", BASE64.encode(.0), .1, .2, .3)]
+    UnexpectedBlobMeta(Vec<u8>, String, u32, u32),
+
     #[error("failure using the NAR writer: {0}")]
     NARWriterError(std::io::Error),
 }