Add Validation Check 2/n: Add Filter Assertion on Writebackfilter

Summary:
## Summary
Add an XCHECK assertion to prevent two WritebackFilters from being registered consecutively in `HTTPTransactionHandlerFilterSeries`. This is a defensive measure to prevent bugs like SEV S570156, where two `CacheRuleResponse` rules used back-to-back in a request handler caused response corruption.

https://fb.workplace.com/groups/proxygendevelopers/permalink/3748394398801257/

Differential Revision: D89745261

fbshipit-source-id: a67c314b0e148cacc99bc32707ba117d4a25cb31

Author: Will Wang

third-party/proxygen/src/proxygen/lib/http/test/MockHTTPMessageFilter.h

@@ -10,6 +10,7 @@
 
 #include <gmock/gmock.h>
 
+#include "proxygen/facebook/revproxy/caching/filter/FilterNames.h"
 #include <proxygen/lib/http/HTTPMessageFilters.h>
 
 namespace proxygen {
@@ -111,4 +112,17 @@ class MockHTTPMessageFilter : public HTTPMessageFilter {
   std::shared_ptr<const HTTPHeaders> requestTrailersCopy_;
 };
 
+// A mock filter that returns kWritebackFilterName for testing
+// the consecutive WritebackFilter check.
+class MockWritebackFilter : public HTTPMessageFilter {
+ public:
+  [[nodiscard]] std::string_view getFilterName() const noexcept override {
+    return filter::kWritebackFilterName;
+  }
+
+  [[noreturn]] std::unique_ptr<HTTPMessageFilter> clone() noexcept override {
+    LOG(FATAL) << "clone() not implemented for MockWritebackFilter";
+  }
+};
+
 } // namespace proxygen