✨ feat: implement input validation for API v2

- Added go-playground/validator dependency

- Created pkg/validation/ package with custom validator wrapper

- Implemented request validation for v2 greet endpoint

- Added structured validation error responses

- Extended BDD tests to cover validation scenarios

- Updated AGENTS.md with v2 API documentation

- Created ADR 0011-validation-library-selection.md

- Simplified server handler creation code

- Updated CHANGELOG with implementation details

pkg/validation/validator.go

@@ -0,0 +1,123 @@
+package validation
+
+import (
+	"DanceLessonsCoach/pkg/config"
+	"errors"
+	"fmt"
+	"reflect"
+	"strings"
+
+	"github.com/go-playground/locales/en"
+	en_translations "github.com/go-playground/validator/v10/translations/en"
+	"github.com/go-playground/validator/v10"
+	ut "github.com/go-playground/universal-translator"
+)
+
+// Validator wraps the go-playground/validator with custom error handling
+type Validator struct {
+	validate   *validator.Validate
+	translator ut.Translator
+}
+
+// NewValidator creates a new validator instance with custom error handling
+func NewValidator() (*Validator, error) {
+	// Create validator instance
+	validate := validator.New()
+
+	// Register translations
+	english := en.New()
+	uni := ut.New(english, english)
+	trans, _ := uni.GetTranslator("en")
+
+	// Register translation for default validator
+	if err := en_translations.RegisterDefaultTranslations(validate, trans); err != nil {
+		return nil, fmt.Errorf("failed to register translations: %w", err)
+	}
+
+	// Register custom validations
+	if err := registerCustomValidations(validate); err != nil {
+		return nil, fmt.Errorf("failed to register custom validations: %w", err)
+	}
+
+	return &Validator{
+		validate:   validate,
+		translator: trans,
+	}, nil
+}
+
+// Validate validates a struct and returns validation errors
+func (v *Validator) Validate(structObj interface{}) error {
+	if structObj == nil {
+		return errors.New("validation: nil object provided")
+	}
+
+	// Get the type of the struct
+	structType := reflect.TypeOf(structObj)
+	if structType.Kind() != reflect.Struct {
+		return fmt.Errorf("validation: expected struct, got %s", structType.Kind())
+	}
+
+	// Perform validation
+	if err := v.validate.Struct(structObj); err != nil {
+		if _, ok := err.(*validator.InvalidValidationError); ok {
+			return fmt.Errorf("validation: invalid validation error: %w", err)
+		}
+
+		// Convert validation errors to custom format
+		validationErrors := err.(validator.ValidationErrors)
+		return v.formatValidationErrors(validationErrors)
+	}
+
+	return nil
+}
+
+// formatValidationErrors converts validator.ValidationErrors to a custom error type
+func (v *Validator) formatValidationErrors(errors validator.ValidationErrors) error {
+	var errorMessages []string
+
+	for _, err := range errors {
+		field := err.Field()
+		tag := err.Tag()
+		param := err.Param()
+
+		// Get custom error message
+		message := fmt.Errorf("%s failed validation for '%s'", field, tag).Error()
+		
+		// Add parameter if available
+		if param != "" {
+			message += fmt.Sprintf(" (parameter: %s)", param)
+		}
+
+		errorMessages = append(errorMessages, message)
+	}
+
+	return &ValidationError{
+		Messages: errorMessages,
+	}
+}
+
+// ValidationError represents multiple validation errors
+type ValidationError struct {
+	Messages []string
+}
+
+func (e *ValidationError) Error() string {
+	return strings.Join(e.Messages, "; ")
+}
+
+// registerCustomValidations registers any custom validation functions
+func registerCustomValidations(validate *validator.Validate) error {
+	// Add custom validations here as needed
+	// Example:
+	// if err := validate.RegisterValidation("custom_tag", customValidationFunc); err != nil {
+	//     return err
+	// }
+	return nil
+}
+
+// GetValidatorFromConfig creates a validator instance based on application config
+func GetValidatorFromConfig(cfg *config.Config) (*Validator, error) {
+	// For now, config doesn't affect validator creation
+	// But this allows future configuration (e.g., language, strict mode)
+	return NewValidator()
+}