refactor: enable readability-container-contains clang-tidy rule

Replace the last few instances of `.count() != 0` and `.count() == 0` and `.count()` patterns with the more expressive C++20 `.contains()` method:

* `std::set<std::string>` in `getblocktemplate` RPC;
* `std::map<std::string, ...>` in `transaction_tests`;
* other bare `std::unordered_set` and `std::map` count calls.

With no remaining violations, enable the `readability-container-contains`
clang-tidy check to prevent future regressions.

src/wallet/test/fuzz/scriptpubkeyman.cpp

@@ -125,7 +125,7 @@ FUZZ_TARGET(scriptpubkeyman, .init = initialize_spkm)
             [&] {
                 const CScript script{ConsumeScript(fuzzed_data_provider)};
                 if (spk_manager->IsMine(script)) {
-                    assert(spk_manager->GetScriptPubKeys().count(script));
+                    assert(spk_manager->GetScriptPubKeys().contains(script));
                 }
             },
             [&] {