Return ErrResponseTooLarge instead of truncating response body

WithMaxResponseBody wrapped the body in io.LimitedReader, which returns EOF
at the cap, so Bytes/JSON/XML silently returned a truncated body with a nil
error despite the documented contract. Read one byte past the limit and
return the new ErrResponseTooLarge sentinel when exceeded; bodies exactly at
the limit still succeed.

error.go

@@ -1,6 +1,7 @@
 package httpx
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
 
@@ -18,6 +19,11 @@ var (
 	ErrNoHealthy      = balancer.ErrNoHealthy
 )
 
+// ErrResponseTooLarge is returned when reading a response body that exceeds
+// the limit configured via WithMaxResponseBody. Any bytes read up to the
+// limit are returned alongside the error.
+var ErrResponseTooLarge = errors.New("httpx: response body exceeds configured limit")
+
 // Error provides structured error information for failed HTTP operations.
 type Error struct {
 	// Op is the operation that failed (e.g. "Get", "Do").