🧹 fix: harden request-body size handling and error unwrapping

Tighten oversized request handling across relay paths and make error matching reliable.

- Align `MAX_REQUEST_BODY_MB` fallback to `32` in request body reader and decompression middleware
- Stop ignoring `GetRequestBody` errors in relay retry paths; return consistent **413** on oversized bodies (400 for other read errors)
- Add `Unwrap()` to `types.NewAPIError` so `errors.Is/As` can match wrapped underlying errors
- `go test ./...` passes

types/error.go

@@ -94,6 +94,14 @@ type NewAPIError struct {
 	StatusCode     int
 }
 
+// Unwrap enables errors.Is / errors.As to work with NewAPIError by exposing the underlying error.
+func (e *NewAPIError) Unwrap() error {
+	if e == nil {
+		return nil
+	}
+	return e.Err
+}
+
 func (e *NewAPIError) GetErrorCode() ErrorCode {
 	if e == nil {
 		return ""