fix: cap log query limit and validate direction parameter

Prevent unbounded memory usage by capping the limit parameter to 5000.
Validate direction against allowed values instead of passing through
to Loki unchecked.

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

internal/monitoring/handlers.go

@@ -589,9 +589,15 @@ func makeQueryLogsHandler(loki *LokiClient) mcp.ToolHandler {
 		if l, ok := args["limit"].(float64); ok && l > 0 {
 			limit = int(l)
 		}
+		if limit > 5000 {
+			limit = 5000
+		}
 
 		direction := "backward"
 		if d, ok := args["direction"].(string); ok && d != "" {
+			if d != "backward" && d != "forward" {
+				return mcp.ErrorContent(fmt.Errorf("direction must be 'backward' or 'forward'")), nil
+			}
 			direction = d
 		}