Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: simongdavies

src/hyperlight-js/src/sandbox/host_fn.rs

@@ -202,9 +202,11 @@ impl HostModule {
                 // data is not supported — reject if blobs are present.
                 if !blobs.is_empty() {
                     return Err(crate::HyperlightError::Error(format!(
-                        "Function '{name}' received {} binary argument(s) but was registered \
-                         with `register` (typed JSON-only). Use `register_js` for functions \
-                         that accept Uint8Array/Buffer arguments.",
+                        concat!(
+                            "Function '{name}' received {} binary argument(s) but was registered ",
+                            "with `register` (typed JSON-only). Use `register_js` for functions ",
+                            "that accept Uint8Array/Buffer arguments."
+                        ),
                         blobs.len()
                     )));
                 }

src/js-host-api/README.md

@@ -465,8 +465,9 @@ Node.js code.
 Host functions natively support `Uint8Array`/`Buffer` arguments and returns.
 Binary data travels through a dedicated sidecar channel, keeping overhead
 minimal. Top-level `Buffer` arguments and returns are passed as raw bytes
-with no encoding. Nested Buffers inside returned objects/arrays are
-serialized by napi-rs's default conversion.
+with no encoding. Nested `Buffer`/`Uint8Array` values inside returned objects
+and arrays are also extracted into the binary sidecar and restored on the
+guest side.
 
 ```javascript
 const { SandboxBuilder } = require('@hyperlight/js-host-api');

src/js-host-api/src/lib.rs

@@ -1200,7 +1200,9 @@ unsafe fn napi_to_json_with_buffer_extraction(
             }
             Ok(serde_json::Value::Object(obj))
         }
-        // Symbols, functions, bigints, external — produce null like JSON.stringify.
+        // Symbols, functions, bigints, external — currently coerced to JSON null for host
+        // return values. Note: this intentionally differs from JSON.stringify, which e.g.
+        // throws on BigInt and omits certain properties instead of serializing them to null.
         _ => Ok(serde_json::Value::Null),
     }
 }