Add IEnumerable<Error> extension methods for simplified AggregateError creation

[Copilot]

This PR introduces convenient extension methods for IEnumerable<Error> to simplify the creation of AggregateError instances, addressing the need for better error aggregation patterns.

Changes Made

New Extension Methods

• ToAggregateError() - Creates an AggregateError with the default message "Multiple errors occurred"
• ToAggregateError(string message) - Creates an AggregateError with a custom message

Usage Examples

// Simple aggregation with default message
var validationErrors = new[]
{
    new LogicError("Name is required"),
    new LogicError("Email format is invalid"),
    new LogicError("Age must be positive")
};

var aggregateError = validationErrors.ToAggregateError();
// Creates AggregateError with message "Multiple errors occurred"

// Custom message for business context
var businessErrors = new[]
{
    new LogicError("Insufficient balance"),
    new LogicError("Account suspended")
};

var customError = businessErrors.ToAggregateError("Payment validation failed");
// Creates AggregateError with custom message

Validation Scenarios

public Result<User> ValidateUser(UserInput input)
{
    var errors = new List<Error>();
    
    if (string.IsNullOrEmpty(input.Name))
        errors.Add(new LogicError("Name is required"));
        
    if (!IsValidEmail(input.Email))
        errors.Add(new LogicError("Email format is invalid"));
    
    if (errors.Any())
        return Result.Failure<User>(errors.ToAggregateError());
        
    return Result.Success(new User(input.Name, input.Email));
}

Files Added/Modified

• New: src/Monads/Error/ErrorEnumerableExtensions.cs - Extension methods implementation
• New: test/Monads.UnitTests/ErrorTests/ErrorEnumerableExtensionsTests.cs - Comprehensive test suite (9 tests)
• Updated: README.md - Added new section "IEnumerable Extensions" with examples and updated Table of Contents

Testing

• All existing tests continue to pass (336 total tests)
• Added 9 new unit tests covering various scenarios including null handling, empty collections, mixed error types, and custom messages
• Manual verification confirms extension methods work as expected

The implementation follows existing patterns in the codebase and provides a clean, minimal API for error aggregation scenarios.

Fixes #10.

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