region_request: log unknown region error for debugging

When regionErrorToLabel returns "unknown", log the full error content
along with the RPC context at INFO level. This helps identify new
error types that should be handled when "unknown" appears in metrics.

The log is placed in onRegionError (the caller) rather than
regionErrorToLabel to:
1. Keep regionErrorToLabel as a pure function without side effects
2. Include RPC context information for better debugging

🤖 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

@@ -1926,6 +1926,13 @@ func (s *RegionRequestSender) onRegionError(
 	}
 
 	regionErrLabel := regionErrorToLabel(regionErr)
+	if regionErrLabel == "unknown" {
+		logutil.Logger(bo.GetCtx()).Info(
+			"unknown region error",
+			zap.Stringer("error", regionErr),
+			zap.Stringer("ctx", ctx),
+		)
+	}
 	metrics.TiKVRegionErrorCounter.WithLabelValues(regionErrLabel, storeIDLabel(ctx)).Inc()
 	if s.Stats != nil {
 		s.Stats.RecordRPCErrorStats(regionErrLabel)