fuzz: check http_request body matches framing

The http_request target asserted that ReadBody() returns an empty
string. That held for the libevent-based http_libevent::HTTPRequest,
where the harness only parsed the request line and headers and never
populated a body. Commit 9c20859b5f (PR #35182) replaced libevent with
http_bitcoin::HTTPRequest, and the target was switched over in
e427c227fa; its LoadBody() now decodes Content-Length and chunked
bodies per RFC 9112, so any fully-parsed request carrying a body trips
the stale assertion (e.g. "POST / HTTP/1.1\r\nContent-Length: 3\r\n\r\nabc").

Replace the emptiness check with a framing-consistency check that
mirrors LoadBody()'s own branch logic: a chunked body is bounded by
MAX_BODY_SIZE, a Content-Length body is exactly that many bytes, and a
request with neither framing header has no body. This strengthens the
target instead of dropping the assertion.

src/test/fuzz/http_request.cpp

@@ -45,6 +45,24 @@ FUZZ_TARGET(http_request)
     (void)http_request.GetHeader(header);
     (void)http_request.WriteHeader(std::string(header), fuzzed_data_provider.ConsumeRandomLengthString(16));
     (void)http_request.GetHeader(header);
+    // Reaching here means LoadControlData/LoadHeaders/LoadBody all succeeded, so the
+    // parsed body must be consistent with the message framing. Before libevent was
+    // replaced with http_bitcoin::HTTPRequest (#35182), ReadBody() always returned an
+    // empty string here; LoadBody now populates the body per RFC 9112 framing, so mirror
+    // its branch logic to assert the body matches the framing that produced it.
     const std::string body = http_request.ReadBody();
-    assert(body.empty());
+    const auto [has_transfer_encoding, transfer_encoding] = http_request.GetHeader("Transfer-Encoding");
+    const auto [has_content_length, content_length] = http_request.GetHeader("Content-Length");
+    if (has_transfer_encoding && ToLower(transfer_encoding) == "chunked") {
+        // A chunked body is the concatenation of the decoded chunks, bounded by MAX_BODY_SIZE.
+        assert(body.size() <= http_bitcoin::MAX_BODY_SIZE);
+    } else if (has_content_length) {
+        // A Content-Length body is exactly that many bytes.
+        const auto parsed_length{ToIntegral<uint64_t>(content_length)};
+        assert(parsed_length);
+        assert(body.size() == *parsed_length);
+    } else {
+        // Absent both framing headers there is no body.
+        assert(body.empty());
+    }
 }