feat: implement core workflow runtime components and unit tests

[cdavernas]

This pull request introduces several structural and code quality improvements to the builder classes in the ServerlessWorkflow.Sdk.Builders namespace, and also updates solution configuration files. The changes focus on making builder classes sealed, simplifying field usage, improving encapsulation, and moving towards a new solution format.

Builder Class Improvements:

• All builder classes such as AuthenticationPolicyDefinitionBuilder, BackoffStrategyDefinitionBuilder, BasicAuthenticationSchemeDefinitionBuilder, and BearerAuthenticationSchemeDefinitionBuilder are now marked as sealed for better encapsulation and to prevent inheritance where not intended. [1] [2] [3] [4]
• Internal state in these builders is now managed with private fields instead of protected properties, reducing surface area and clarifying intent. [1] [2] [3] [4]
• Method signatures have been simplified by removing the virtual keyword where overriding is not required, and by using direct field access instead of property setters/getters. [1] [2] [3] [4] [5]
• The Build() methods have been updated for clarity and correctness, including improved null checks and direct assignment from private fields. [1] [2] [3] [4] [5]

Solution and Configuration Updates:

• The legacy Visual Studio solution file (ServerlessWorkflow.Sdk.sln) has been removed, and a new solution file in .slnx format (ServerlessWorkflow.Sdk.slnx) has been added, reflecting a migration to a new project structure and tooling. [1] [2]
• A new .claude/settings.local.json file has been added to specify allowed shell commands and permissions, likely for local development or automation tooling.

These changes collectively improve code maintainability, safety, and project organization.