.NET: AddAIAgent - Rename or add overload

AddAIAgent extension methods in the Hosting package implicitly register an `AIAgent` as a `KeyedSingleton`

https://github.com/microsoft/agent-framework/blob/518fd447fd259581ae58e17aae66f8be50e9cbe1/dotnet/src/Microsoft.Extensions.AI.Agents.Hosting/HostApplicationBuilderAgentExtensions.cs#L100-L120

When resolving the agent using DI, it's not obvious you need to use `FromKeyedServices` to get the agent. Using `FromServices` throws.

i.e.

**Works**

```csharp
app.MapGet("/chat", async ([FromKeyedServices("user-agent")] AIAgent agent, string question) =>
{
AgentRunResponse agentResponse = await agent.RunAsync(question);

return Results.Ok(agentResponse.Text);
});
```

**Error**

```csharp
app.MapGet("/chat", async ([FromServices] AIAgent agent, string question) =>
{
AgentRunResponse agentResponse = await agent.RunAsync(question);

return Results.Ok(agentResponse.Text);
});
```

Suggestions:

1. Rename existing `AddAIAgent` to `AddKeyedAIAgent`
2. Add overloads for both `AddAIAgent` and `AddKeyedAIAgent`

I'd be in favor of option 2.