[Feat]: Refactor `A2ARouteBuilderExtensions` to leverage dependency injection

### Is your feature request related to a problem? Please describe.

Currently, all methods in `A2ARouteBuilderExtensions` require explicitly passing an `ITaskManager`. This feels awkward and clutters otherwise clean .NET application code.

### Describe the solution you'd like

I’d like `A2ARouteBuilderExtensions` to resolve the `ITaskManager` from the request’s `IServiceProvider`. As far as I know, it’s already established that a server application can only host a single agent, due to restrictions around well-known documents that must be rooted. Consequently, we can safely assume only one `ITaskManager` will ever be registered—likely through a new `IServiceCollection.AddA2A` extension. This makes dependency injection the natural choice.

With this change, we could eliminate the awkward Attach pattern and instead allow handler registration directly in the agent’s constructor, resulting in cleaner, more idiomatic .NET code.

### Describe alternatives you've considered

Doing something ugly like the following:

```c#
var taskManager = new TaskManager(); // <= this should have been registered/constructured through DI instead
var builder = WebApplication.CreateBuilder(args);
builder.Services.ConfigureHttpJsonOptions(options =>
{
options.SerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull;
});
builder.Services.AddSingleton(taskManager);
builder.Services.AddSingleton();

var app = builder.Build();
app.Services.GetRequiredService(); // <- this ugly trick is required, so that my agent is actually constructured before someone hits the discovery endpoint, thus registering the TaskManager's handlers
app.MapA2A(taskManager, "/chat"); // <= this shouldn't require passing the TaskManager, and differs from 99% of .NET extensions
app.MapWellKnownAgentCard(taskManager, "/chat"); // <= this shouldn't require passing the TaskManager, and differs from 99% of .NET extensions
await app.RunAsync();
```

```c#
public class Agent
{

ITaskManager _taskManager;

public Agent(ITaskManager taskManager)
{
_taskManager = taskManager;
_taskManager.OnTaskCreated = ExecuteTaskAsync;
_taskManager.OnTaskUpdated = ExecuteTaskAsync;
_taskManager.OnMessageReceived = ProcessMessageAsync;
_taskManager.OnTaskCancelled = CancelTaskAsync;
_taskManager.OnAgentCardQuery = GetCardAsync;
}

//Omitted for brevity

}
```

### Additional context

_No response_

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