Add mergeability failure reasons to merge train errors

What does this MR do and why?

Removes the merge_request_detailed_failure_reasons feature flag, which has been rolled out and is stable, and keeps its behavior enabled.

Merge train validation now always runs the mergeability checks and includes the reason for each failed check in the Merge request is not mergeable error, instead of falling back to a generic message.

Changes:

• Delete the merge_request_detailed_failure_reasons flag definition.
• Drop the flag conditionals in MergeRequests::MergeStrategies::FromTrainRef (the detailed-reasons path is now the only path).
• Remove the obsolete "flag disabled" spec context.

Closes #594769 (closed)