Remove sealed modifier from CpSolver and implement standard dispose pattern

[alirezanet]

Problem

The sealed modifier added to CpSolver in commit f2e2e49 breaks existing code that inherits from CpSolver. The sealed keyword was not necessary for implementing IDisposable and prevents legitimate inheritance scenarios.

Solution

This PR:

• Removes the sealed modifier from the CpSolver class
• Implements the standard .NET dispose pattern with protected virtual Dispose(bool disposing)
• Adds XML documentation for the dispose methods
• Maintains full backward compatibility for non-inheriting code

Benefits

1. Allows inheritance: Users can now extend CpSolver for custom solver implementations
2. Standard pattern: Follows the recommended .NET IDisposable pattern for classes that may be inherited
3. Safe disposal: Derived classes can override Dispose(bool disposing) to add their own cleanup logic
4. No breaking changes: Existing non-inheriting code continues to work exactly as before

Example Usage

With this change, users can safely inherit from CpSolver:

public class CustomCpSolver : CpSolver
{
    private SomeResource _customResource;

    protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            _customResource?.Dispose();
        }
        base.Dispose(disposing);
    }
}

Related to commit: f2e2e49

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.