[New article]: Add how-to that shows how to switch model providers with Microsoft.Extensions.AI

### Proposed topic or title

How to switch model providers with Microsoft.Extensions.AI

### Location in table of contents.

https://github.com/dotnet/docs/tree/main/docs/ai/how-to

### Reason for the article

Existing articles assume you're working with a single provider. However, there are scenarios where you might want or need to use different providers.

Scenarios:

- Local testing vs Production deployment
- Smart routing (choosing a different provider based on query complexity)
- Evaluations (batch testing prompts using different model providers)

### Article abstract

https://devblogs.microsoft.com/dotnet/ai-vector-data-dotnet-extensions-ga/#portability-across-model-and-vector-store-providers

Expand on this article, specifically these sections.

```csharp
IChatClient chatClient =
environment == "Development"
? new OllamaApiClient("YOUR-OLLAMA-ENDPOINT", "qwen3")
: new AzureOpenAIClient("YOUR-AZURE-OPENAI-ENDPOINT", new DefaultAzureCredential())
.GetChatClient("gpt-4.1")
.AsIChatClient();

await foreach (var message in chatClient.GetStreamingResponseAsync("What is AI?"))
{
Console.Write($"{message.Text}");
};

IEmbeddingGenerator> embeddingGenerator =
environment == "Development"
? new OllamaApiClient("YOUR-OLLAMA-ENDPOINT", "all-minilm")
: new AzureOpenAIClient("YOUR-AZURE-OPENAI-ENDPOINT", new DefaultAzureCredential())
.GetEmbeddingClient("text-embedding-3-small")
.AsIEmbeddingGenerator();

var embedding = await embeddingGenerator.GenerateAsync("What is AI?");
```

### Relevant searches

- How to use multiple model providers with Microsoft.Extensions.AI
- Working with local and hosted models with Microsoft.Extensions.AI
- Switch model providers with Microsoft.Extensions.AI