fix(bifrost): synchronize signer constants cache reads

Concurrent signings can read requestHeight while another signing updates the constants cache. This causes race-detector failures in the required Bifrost race job added by !5087 (merged).

Read the cached height and churn interval together under the existing mutex, then release it before fetching. The refresh logic stays the same.

Adds a test with 32 concurrent callers, covering both first use and an expired cache.