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/bypass.go

@@ -122,10 +122,6 @@ func (c *checkBypass) modifyResponse(resp *http.Response) error {
 	return c.modRes.modifyResponse(resp)
 }
 
-func (c *checkBypass) requiresBodyRewrite() bool {
-	return requiresBodyRewrite(c.modRes)
-}
-
 func (m *Middleware) withCheckBypass() any {
 	if len(m.Bypass) > 0 {
 		modReq, _ := m.impl.(RequestModifier)