[validation] Annotate ChainstateManager::m_best_header as guarded by cs_main

Summary:
This is a partial backport of core#26140
https://github.com/bitcoin/bitcoin/pull/26140/commits/1d87137227ee125018006935d55164a72265c935
Depends on D15287

Test Plan:
With Debug:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15288