Refactor 'AI Backend' -> 'AI Service'; Move AI Namespace to semantic_kernel.connectors.ai

[awharrison-28]

Motivation and Context

This PR is a consistency update between the Python and .NET Semantic Kernels.

This PR does not contain any feature enhancements or improvements!

Description

• Replace terminology 'backend' with 'service'
• Move semantic_kernel.ai (semantic_kernel/ai) to semantic_kernel.connectors.ai (semantic_kernel/connectors/ai)

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: