Tag the deploy step span with the service host kind for per-service deploy telemetry

## Summary

The deploy step span emitted by the up/deploy execution graph does not carry the service **host kind**, so telemetry cannot reliably attribute a specific deploy step to its host (e.g. `azure.ai.agent`) in multi-service projects. Adding the host to the deploy step's tags (one line) makes per-host deploy success/failure metrics queryable across single- and multi-service projects.

## Background

For each service, the up/deploy graph creates a deploy step:

```go
// cli/azd/internal/cmd/service_graph.go (~line 517)
deployStepName := "deploy-" + svc.Name
...
g.AddStep(&exegraph.Step{
Name: deployStepName, // "deploy-" (service name, not host)
Tags: []string{"deploy"}, // no host kind
...
})
```

The scheduler emits an `exegraph.step` span per step with `exegraph.step.name`, `exegraph.step.tags`, and per-step `Success`/`ResultCode` (`cli/azd/pkg/exegraph/scheduler.go`).

Neither `serviceManager.Deploy` (`cli/azd/pkg/project/service_manager.go`) nor the extension bridge `ExternalServiceTarget.Deploy` (`cli/azd/pkg/project/service_target_external.go`) starts a span or sets a host attribute, and the root command span's `project.service.hosts` / `project.service.targets` are independently sorted **sets** with no service-name -> host mapping (`cli/azd/pkg/project/project.go`, `cli/azd/pkg/project/project_manager.go`).

## Impact

Today telemetry/dashboards can determine that a project contains an `azure.ai.agent` service and that a `deploy-*` step succeeded or failed, but cannot tie a specific deploy step to its host kind. Per-host deploy reliability metrics (for example, the Foundry hosted-agent deploy-step reliability metric in the agents telemetry data contract) are therefore only clean for single-service projects; multi-service projects conflate the agent's deploy step with other services' deploy steps.

## Proposed fix (one line)

Include the host kind in the deploy step tags:

```go
// cli/azd/internal/cmd/service_graph.go
Tags: []string{"deploy", string(svc.Host)},
```

This makes `exegraph.step.tags` equal to e.g. `["deploy","azure.ai.agent"]`, so a query like
`Properties['exegraph.step.tags'] has "azure.ai.agent"` identifies the agent's deploy step directly, in single- and multi-service projects.

Alternative: add a dedicated singular `project.service.host` attribute on the deploy step span for cleaner grouping (instead of overloading tags).

## Notes

- Host kinds are low-cardinality and non-PII (e.g. `containerapp`, `appservice`, `azure.ai.agent`), so they are safe to emit as telemetry.
- Consider doing the same for the `package-` / `publish-` steps for symmetry.
- The new tag value / attribute may need classification in the Data Catalog portal before it appears in dashboards.