Build Duo Agent Platform session URLs with Gitlab::UrlBuilder

Fixes https://new-sentry.gitlab.net/organizations/gitlab/issues/3973765/

What does this MR do and why?

GraphQL queries for webUrl on a DuoWorkflow were raising NotImplementedError: No URL builder defined for #<Ai::DuoWorkflows::Workflow ...>: Types::Ai::DuoWorkflows::WorkflowType implements Types::TodoableInterface (which declares field :web_url) and presents through WorkflowPresenter, so the field resolved via Gitlab::View::Presenter::Base#web_url → Gitlab::UrlBuilder.build, which had no case for a workflow.

While this MR was open, !248587 (merged) added an explicit WorkflowPresenter#web_url that delegates to the model, which stops the exception. So the crash is already fixed on master.

What is left here is the underlying gap: Gitlab::UrlBuilder still does not know how to build a URL for a session, so the presenter needs its own override, and every other caller builds the URL by hand. This MR adds the workflow case to the EE URL builder and makes Ai::DuoWorkflows::Workflow#web_url delegate to it. The builder uses the named project_automate_agent_session route instead of appending the ID to the index route, and returns nil for namespace-level sessions, matching the model's previous behaviour.

The generated URL is byte-identical to the previous string concatenation, so nothing user-facing changes.

How to set up and validate locally

In a Rails console, with a project-level session:

workflow = Ai::DuoWorkflows::Workflow.last
Gitlab::UrlBuilder.build(workflow)
# => "http://gdk.test:3000/<project_full_path>/-/automate/agent-sessions/<id>"
Gitlab::UrlBuilder.build(workflow, only_path: true)
# => "/<project_full_path>/-/automate/agent-sessions/<id>"

On master, the first call raises NotImplementedError.

MR acceptance checklist

• Tested locally: ee/spec/lib/ee/gitlab/url_builder_spec.rb (72 examples), the #web_url block in ee/spec/models/ai/duo_workflows/workflow_spec.rb, ee/spec/presenters/ai/duo_workflows/workflow_presenter_spec.rb, ee/spec/graphql/types/ai/duo_workflows/workflow_type_spec.rb, and the specs of the remaining workflow.web_url callers (start_workflow_service, Slack and GitLab Duo note messaging adapters) — all pass.