fuzz: add a target for DifferenceFormatter Class

Add fuzz test to verify that arbitrary input successfully deserialized
by DifferenceFormatter will maintain the sorted-without-duplicates invariant.

src/test/fuzz/CMakeLists.txt

@@ -41,6 +41,7 @@ add_executable(fuzz
   decode_tx.cpp
   descriptor_parse.cpp
   deserialize.cpp
+  difference_formatter.cpp
   eval_script.cpp
   feefrac.cpp
   fee_rate.cpp

src/test/fuzz/difference_formatter.cpp

@@ -0,0 +1,32 @@
+// Copyright (c) 2025 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <blockencodings.h>
+#include <streams.h>
+#include <random.h>
+#include <test/fuzz/fuzz.h>
+
+#include <vector>
+
+FUZZ_TARGET(difference_formatter)
+{
+    const auto block_hash = InsecureRandomContext{{}}.rand256();
+    DataStream ss{};
+    ss << block_hash << std::span{buffer};
+
+    // Test deserialization
+    try {
+        BlockTransactionsRequest test_container;
+        ss >> test_container;
+        assert(test_container.blockhash == block_hash);
+
+        // Invariant: strictly monotonic increasing (no duplicates allowed)
+        for (size_t i = 1; i < test_container.indexes.size(); ++i) {
+            assert(test_container.indexes[i] > test_container.indexes[i-1]);
+        }
+
+    } catch (const std::ios_base::failure&) {
+        // Expected for malformed input
+    }
+}