Setup OpNode.Core and OpNode.Core.Tests projects with validation components

[Copilot]

This PR establishes the foundational architecture for OpNode by creating clean, testable core projects completely decoupled from UI dependencies as outlined in Issue #65.

What's New

OpNode.Core Class Library

• Modern .NET 8.0 class library with zero external dependencies
• ValidationResult class providing structured validation responses with static factory methods
• NamingValidator service offering comprehensive naming validation including:
  • Empty/null/whitespace checking
  • Length constraints (minimum/maximum)
  • Character validation (alphanumeric + underscores only)
  • Comprehensive validation combining all rules

OpNode.Core.Tests Unit Test Project

• MSTest-based test project with full coverage of core components
• 13 comprehensive unit tests covering all validation scenarios and edge cases
• Demonstrates proper usage patterns for the validation components

Architecture Highlights

The implementation provides a clean foundation that:

• Has zero UI dependencies (no System.Windows.Forms or similar references)
• Uses modern C# features like nullable reference types and init-only properties
• Follows service-based architecture patterns ready for expansion
• Maintains complete separation between business logic and presentation layers

Usage Example

var validator = new NamingValidator();
var result = validator.ValidateName("Valid_Name123", minLength: 3, maxLength: 50);

if (result.IsValid)
{
    // Proceed with valid name
}
else
{
    Console.WriteLine($"Validation failed: {result.ErrorMessage}");
}

Verification

• ✅ Clean build with zero warnings or errors
• ✅ All 13 unit tests pass successfully
• ✅ Zero external package dependencies confirmed
• ✅ Zero project references to UI libraries confirmed
• ✅ Complete documentation and examples provided

This establishes the testable core architecture foundation required by Epic #64, enabling future development of validation, operations, and serialization components without UI coupling.

Fixes #65.

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