Skip mergeability cache during auto-merge process

Summary

When auto-merge processes an MR to decide whether to trigger the actual merge, bypass the mergeability cache to ensure fresh approval state. This prevents queueing merge attempts based on stale cached data, avoiding unnecessary MergeWorker jobs that would just be rejected by the merge strategy's own fresh check.

What changes

• MergeWhenChecksPassService#process — passes use_cache: false to mergeable?
• AddToMergeTrainWhenChecksPassService#process — passes use_cache: false to mergeable?
• Both gated behind :automerge_skip_mergeability_cache feature flag (per-project)

What doesn't change

• The availability_details check (can automerge be set?) still uses the cache — staleness there has no impact on merge correctness
• The merge strategies (FromSourceBranch, FromTrainRef) already pass use_cache: false independently

Related

• Depends on !227379 (short-TTL approval cache)

Feature flag

:automerge_skip_mergeability_cache — default disabled, per-project actor

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.