.NET: Simplify Anthropic Foundry sample TokenCredential impl, removing unnecessary complexity.

[rogerbarreto]

Motivation and Context

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR simplifies the AnthropicAzureTokenCredential implementation in the Anthropic sample by removing token caching logic and converting to a primary constructor pattern. While this reduces code complexity, it introduces performance concerns.

Key changes:

• Converted from traditional constructor to C# primary constructor syntax
• Removed token caching, lock mechanism, and token expiration handling
• Simplified the Apply method to directly fetch tokens on each invocation