Refuse a yarn lockfile the scanner stopped in, and a forge answer the cap cut

Either was read as whole and could leave a pinned version unnamed (review of #128).

modules/gitea/cmd/npm-registry/forge.go

@@ -78,6 +78,9 @@ func (f *Forge) password() (string, error) {
 	return strings.TrimSpace(string(raw)), nil
 }
 
+// maxBody is the largest answer read from the forge.
+const maxBody = 32 << 20
+
 // do sends one request and answers the body; 404 is errNotFound.
 func (f *Forge) do(ctx context.Context, method, path string) ([]byte, error) {
 	password, err := f.password()
@@ -95,10 +98,15 @@ func (f *Forge) do(ctx context.Context, method, path string) ([]byte, error) {
 		return nil, fmt.Errorf("the forge did not answer %s %s: %w", method, path, err)
 	}
 	defer res.Body.Close()
-	body, err := io.ReadAll(io.LimitReader(res.Body, 32<<20))
+	// One byte past the cap is read, so a body the cap cut is told from one that fits it exactly: a
+	// cut body is refused, never parsed as if it were whole.
+	body, err := io.ReadAll(io.LimitReader(res.Body, maxBody+1))
 	if err != nil {
 		return nil, err
 	}
+	if len(body) > maxBody {
+		return nil, fmt.Errorf("the forge's answer to %s %s is larger than %d MiB and was not read whole", method, path, maxBody>>20)
+	}
 	switch {
 	case res.StatusCode == http.StatusNotFound:
 		return nil, fmt.Errorf("%s %s: %w", method, path, errNotFound)