region_request: log unknown region error for debugging

When regionErrorToLabel returns "unknown", it means the error type
is not recognized. Add an INFO log to capture the full error content,
which helps identify new error types that should be handled.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ekexium

internal/locate/region_request.go

@@ -1905,6 +1905,7 @@ func regionErrorToLabel(e *errorpb.Error) string {
 	} else if e.GetUndeterminedResult() != nil {
 		return "undetermined_result"
 	}
+	logutil.BgLogger().Info("unknown region error", zap.Stringer("error", e))
 	return "unknown"
 }