Add required flag to parameter description and improve plan creation

[lemillermicrosoft]

This commit adds a "(required)" suffix to the description of any parameter that is marked as required in the OpenAPI schema. This makes it easier for users to see which parameters are mandatory when using the skills.

Additionally, this commit improves the plan creation logic in the chat skill by passing the full context and user intent to the planner, and merging any context variables into the plan's state. This allows the planner to generate more relevant and accurate steps based on the user input and the current situation.

Contribution Checklist

• [ ] The code builds clean without any errors or warnings
• [ ] The PR follows SK Contribution Guidelines (https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
• [ ] The code follows the .NET coding conventions (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions) verified with dotnet format
• [ ] All unit tests pass, and I have added new tests where possible
• [ ] I didn't break anyone :smile: