Add how-to article: Switch model providers with Microsoft.Extensions.AI

[Copilot]

This PR adds a comprehensive how-to article that demonstrates how to dynamically switch between different AI model providers using Microsoft.Extensions.AI. The article addresses the key scenarios outlined in the issue:

New Content

Article: docs/ai/how-to/switch-model-providers.md

• Environment-based switching (development vs production)
• Smart routing based on query complexity
• Batch evaluation across multiple providers
• Configuration-driven provider selection
• Dependency injection patterns with keyed services

Code Examples: Five complete, working examples with project files:

1. Environment switching: Local Ollama for development, Azure OpenAI for production
2. Smart routing: Automatic provider selection based on query characteristics
3. Batch evaluation: Compare responses across multiple providers
4. Configuration-based: JSON-driven provider configuration
5. Dependency injection: Register multiple providers with DI container

Technical Details

All code examples:

• Target .NET 8.0 with compatible package versions
• Build successfully without errors
• Follow Microsoft.Extensions.AI best practices
• Include proper error handling and fallback strategies

The article also includes comprehensive best practices covering:

• Error handling and fallback mechanisms
• Performance monitoring and logging
• Cost optimization strategies
• Provider-specific configuration patterns

Navigation

Updated docs/ai/toc.yml to add a new "How-to guides" section containing the article, properly integrated into the existing documentation structure.

This expands significantly on the blog post content referenced in the issue, providing production-ready examples that developers can immediately implement in their applications.

Fixes #48552.

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