fix(rpc/test): add mempool_sequence to expected getmempoolinfo key set

Commit 4ecf96a added the mempool_sequence field to getmempoolinfo;
core_compat.rs asserts an exact key set and was failing because the
new field was missing from the expected list.

Op: correct
Restores: test:selected_methods_match_core_documented_key_sets

Author: metaphorics

crates/rpc/tests/core_compat.rs

@@ -54,6 +54,7 @@ fn selected_methods_match_core_documented_key_sets() -> Result<(), Box<dyn std::
             "incrementalrelayfee",
             "unbroadcastcount",
             "fullrbf",
+            "mempool_sequence",
         ],
     )?;
     assert_keys(