[elsa] fix(GraphOperator): optimize single-key update handling

[RonnyChan96]

• Add special case handling for keys.length === 1 scenarios
• Skip parent config lookup when updating root-level configurations
• Prevent unnecessary null checks for single-key updates
• Maintain existing multi-key update logic unchanged

Key improvements:

1. Behavior change:

   • Before: Would throw when parentConfig is null for single-key paths
   • Now: Directly updates config without parent lookup for single-key

2. Performance benefit:

   • Eliminates redundant slice(0,-1) operation for single-key cases
   • Avoids unnecessary parent config existence check

3. Safety:

   • Still validates empty keys array (unchanged)
   • Preserves all existing error handling for multi-key paths