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.

response_limit_test.go

@@ -2,6 +2,7 @@ package httpx_test
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"net/http/httptest"
@@ -32,13 +33,30 @@ func TestClient_MaxResponseBody(t *testing.T) {
 		}
 	})
 
-	t.Run("truncates response exceeding limit", func(t *testing.T) {
+	t.Run("returns ErrResponseTooLarge when exceeding limit", func(t *testing.T) {
 		largeBody := strings.Repeat("x", 1000)
 		srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
 			fmt.Fprint(w, largeBody)
 		}))
 		defer srv.Close()
 
+		client := httpx.New(httpx.WithMaxResponseBody(100))
+		resp, err := client.Get(context.Background(), srv.URL+"/")
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if _, err := resp.Bytes(); !errors.Is(err, httpx.ErrResponseTooLarge) {
+			t.Fatalf("err = %v, want ErrResponseTooLarge", err)
+		}
+	})
+
+	t.Run("allows body exactly at limit", func(t *testing.T) {
+		exact := strings.Repeat("x", 100)
+		srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			fmt.Fprint(w, exact)
+		}))
+		defer srv.Close()
+
 		client := httpx.New(httpx.WithMaxResponseBody(100))
 		resp, err := client.Get(context.Background(), srv.URL+"/")
 		if err != nil {
@@ -46,7 +64,7 @@ func TestClient_MaxResponseBody(t *testing.T) {
 		}
 		b, err := resp.Bytes()
 		if err != nil {
-			t.Fatalf("reading body: %v", err)
+			t.Fatalf("reading body at exact limit: %v", err)
 		}
 		if len(b) != 100 {
 			t.Fatalf("body length = %d, want %d", len(b), 100)