Rename Ensure class to Contract to resolve naming conflicts

[konard]

Summary

This PR resolves issue #63 by renaming the Ensure class to Contract to avoid naming conflicts with external libraries like Ensure.That.

Problem

The existing Ensure class conflicts with the popular Ensure.That library, making it difficult for users to use both libraries in the same project. The issue specifically mentioned that this conflict could complicate code translation to C++.

Solution

• Renamed Ensure → Contract across the entire codebase
• Updated extension root classes: EnsureAlwaysExtensionRoot → ContractAlwaysExtensionRoot, EnsureOnDebugExtensionRoot → ContractOnDebugExtensionRoot
• Updated all extension methods to work with the new classes
• Updated both C# and C++ implementations to maintain consistency
• Updated all tests to use the new API

API Changes

Before:

Ensure.Always.ArgumentNotNull(argument, "argumentName");
Ensure.OnDebug.ArgumentNotNull(argument, "argumentName");

After:

Contract.Always.ArgumentNotNull(argument, "argumentName");
Contract.OnDebug.ArgumentNotNull(argument, "argumentName");

C++ Changes:

// Before
Platform::Exceptions::Ensure::Always::ArgumentNotNull(ptr, "ptr");

// After  
Platform::Exceptions::Contract::Always::ArgumentNotNull(ptr, "ptr");

Testing

• ✅ All C# tests pass (2/2)
• ✅ Build succeeds with no errors
• ✅ Maintains same functionality and performance characteristics
• ✅ Extension mechanism works identically

Breaking Changes

This is a breaking change that requires users to update their code from Ensure to Contract. However, the API pattern and functionality remain exactly the same.

Benefits

• ✅ Resolves naming conflicts with external libraries
• ✅ Allows users to use both Platform.Exceptions and Ensure.That together
• ✅ Maintains consistency between C# and C++ implementations
• ✅ Uses a semantically appropriate name (Contract is commonly used for assertion libraries)

Fixes #63

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