Fix flakey cosmos mempool tests

What's most likely happening is these transactions get committed before we're able to check if they're in pending, so the test is failing.

A todo here would be to change this to pass on tx commited OR in pending list.

_Originally posted by @vladjdk in https://github.com/cosmos/evm/pull/856#discussion_r2565595251_

Start by locating the Cosmos mempool tests and reproduce the flaky failure by running them repeatedly. Inspect the pending-transaction check; done means the test passes whether the transaction is committed or remains in the pending list.

Written by the indexing model from the issue text.