fix(middleware): skip body rewriters when buffering fails

Prevent response modifiers that require body rewriting from running when
the body rewrite gate blocks buffering (for example, chunked transfer
encoding).

Add an explicit `requiresBodyRewrite` capability and implement it for
HTML/theme/error-page modifiers, including bypass delegation.

Also add a regression test to ensure the original response body remains
readable and is not closed prematurely when rewrite is blocked.

This commit fixeds the "http: read on closed response body" with empty page error
happens when body-rewriting middleware (like themed) runs on responses where body rewrite is blocked (e.g. chunked),
then the gate restores an already-closed original body.

internal/net/gphttp/middleware/bypass.go

@@ -122,6 +122,10 @@ 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)