Match Go Query: quote JSON keys in output records

Go's ToJson produces valid JSON with quoted keys like {"name":"Alice"}.
Rust was producing invalid JSON with unquoted keys like {name:"Alice"}.

seaweed-volume/src/server/grpc_server.rs

@@ -3393,13 +3393,15 @@ impl VolumeServer for VolumeGrpcService {
                         }
                     }
 
-                    // Build output record: {selection:value,...} (Go's ToJson format)
+                    // Build output record: {"selection":value,...} (Go's ToJson format)
                     records.push(b'{');
                     for (i, sel) in req.selections.iter().enumerate() {
                         if i > 0 {
                             records.push(b',');
                         }
+                        records.push(b'"');
                         records.extend_from_slice(sel.as_bytes());
+                        records.push(b'"');
                         records.push(b':');
                         let val = &parsed[sel];
                         let raw = if val.is_null() {