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/middleware_chain.go

@@ -13,6 +13,10 @@ type middlewareChain struct {
 	modResps []ResponseModifier
 }
 
+type bodyRewriteRequired interface {
+	requiresBodyRewrite() bool
+}
+
 // TODO: check conflict or duplicates.
 func NewMiddlewareChain(name string, chain []*Middleware) *Middleware {
 	chainMid := &middlewareChain{}
@@ -59,6 +63,9 @@ func modifyResponseWithBodyRewriteGate(mr ResponseModifier, resp *http.Response)
 	originalBody := resp.Body
 	originalContentLength := resp.ContentLength
 	allowBodyRewrite := canBufferAndModifyResponseBody(responseHeaderForBodyRewriteGate(resp))
+	if !allowBodyRewrite && requiresBodyRewrite(mr) {
+		return nil
+	}
 
 	if err := mr.modifyResponse(resp); err != nil {
 		return err
@@ -87,6 +94,11 @@ func modifyResponseWithBodyRewriteGate(mr ResponseModifier, resp *http.Response)
 	return nil
 }
 
+func requiresBodyRewrite(mr ResponseModifier) bool {
+	required, ok := mr.(bodyRewriteRequired)
+	return ok && required.requiresBodyRewrite()
+}
+
 func responseHeaderForBodyRewriteGate(resp *http.Response) http.Header {
 	h := resp.Header.Clone()
 	if len(resp.TransferEncoding) > 0 && len(h.Values("Transfer-Encoding")) == 0 {