Add Spring Boot Custom Validation Module with Logback CompatibilityTrigger PR comparison with minor change

[sandipk09roy-pixel]

Closes #1

Summary

This pull request introduces a Spring Boot module that demonstrates custom validation using @Valid, @RequestBody, and BindingResult. It provides a reproducible example for students and collaborators to understand field-level validation and structured error handling in REST APIs.

Features

• UserDTO with annotations: @Email, @Size, @NotBlank, @Min
• UserController with validation logic and error response formatting
• LogbackPing class to ensure compatibility with logback-core packaging
• Entry point via SpringBootCustomValidationApplication.java
• Compatible with Spring Boot 3.2.5 and Java 21

How to Test

curl -X POST http://localhost:8081/register \
     -H "Content-Type: application/json" \
     -d "{\"email\":\"invalid-email\",\"password\":\"short\"}"

Expected response:

[
  "Please provide a valid email address.",
  "Name must not be blank.",
  "Age must be at least 18."
]
Notes
- No external configuration required
- Designed for onboarding and reproducibility
- Ideal for student learning and collaborative extension

[sandipk09roy-pixel]

@sandipk09roy-pixel would love your feedback on this validation module — designed for onboarding and reproducibility.