AddContainer + PublishAsAzureContainerApp does not configure ACR identity for image pull

### Describe the bug

When using `AddContainer("name", "image").PublishAsAzureContainerApp(...)`, the generated Bicep does not include ACR registry credentials or user-assigned managed identity for image pull. The container image is hardcoded as a static string instead of being parameterized. This means the container app cannot pull images from ACR using identity-based auth, unlike `AddProject` which gets this automatically.

The root cause is in `ContainerAppContext.BuildContainerApp()` — ACR parameters are only allocated when `TryGetContainerImageName` returns `false`, but containers always have a `ContainerImageAnnotation`, so it always returns `true`.

### Expected Behavior

When `PublishAsAzureContainerApp` is used on a container resource, the generated Bicep should include parameterized image, ACR registry credentials, and managed identity — matching the behavior of project resources.

### Steps To Reproduce

```csharp
var builder = DistributedApplication.CreateBuilder(args);
builder.AddAzureContainerAppEnvironment("env");

builder.AddContainer("myapp", "myimage:latest")
.PublishAsAzureContainerApp((infra, app) => { });

builder.Build().Run();
```

Inspect the generated Bicep — no `registries` block, no user-assigned identity, image is literal `"myimage:latest"`.

### Anything else?

Our use case is deploying images from legacy projects that push directly to ACR outside of Aspire. We use `AddContainer` to reference those images and `PublishAsAzureContainerApp` to deploy them as container apps, but the generated Bicep lacks the identity-based ACR pull configuration needed to actually pull those images.

Affected code paths:
- `BaseContainerAppContext.TryGetContainerImageName` (`BaseContainerAppContext.cs:89`)
- `ContainerAppContext.BuildContainerApp` (`ContainerAppContext.cs:33`)
- `AddContainerRegistryManagedIdentity` (`BaseContainerAppContext.cs:487`)
- `AddContainerRegistryParameters` (`BaseContainerAppContext.cs:501`)

Start in BaseContainerAppContext.cs at TryGetContainerImageName, AddContainerRegistryManagedIdentity, and AddContainerRegistryParameters, then trace their use from ContainerAppContext.cs in BuildContainerApp. Compare the AddContainer path with the project-resource path and inspect the generated Bicep for the reproduction. Done means the container app has a parameterized image, ACR registry credentials, and user-assigned managed identity for image pulls.

Written by the indexing model from the issue text.