fix(middleware): gate only body response modifiers

Replace the rewrite requirement check with a BodyResponseModifier
marker and treat header and body modifiers separately.

This ensures header/status rewrites still apply when body rewriting is
blocked (for binary, encoded, or chunked responses), while body changes
are skipped safely. It also avoids body reset/close side effects and
returns early on passthrough responses.

Update middleware tests to cover split header/body behavior and themed
middleware body-skip scenarios.

internal/net/gphttp/middleware/themed.go

@@ -54,9 +54,7 @@ func (m *themed) modifyResponse(resp *http.Response) error {
 	return m.m.modifyResponse(resp)
 }
 
-func (*themed) requiresBodyRewrite() bool {
-	return true
-}
+func (*themed) isBodyResponseModifier() {}
 
 func (m *themed) finalize() error {
 	m.m.Target = "body"