Add visitor pattern with course content inspection

[darbyluv2code]

Summary

• Implements Visitor pattern with course content inspection use case
• Demonstrates ContentCountVisitor and TotalStudyTimeVisitor on shared Course structure
• Location: section-04-behavioral-design-patterns/15-visitor/

Pattern Roles

• ContentVisitor: visitor interface declaring visit methods
• CourseContent: element interface
• VideoLesson, Quiz, Assignment: concrete elements
• Course: object structure containing elements
• ContentCountVisitor: concrete visitor counting content by type
• TotalStudyTimeVisitor: concrete visitor summing estimated study time

Test plan

• Run mvn compile in section-04-behavioral-design-patterns/15-visitor/
• Run mvn exec:java or MainApp directly to verify output
• Confirm both visitors produce expected counts and study times