Reduce cache lookups in CCoinsViewCache::FetchCoin
Summary:
Enhanced efficiency and readability of CCoinsViewCache::FetchCoin by replacing separate find() and emplace() calls with a single try_emplace(), reducing map lookups and potential insertions.
This is a backport of core#30326
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D19004