Extend exegraph to handle custom workflow processing

## Context
Custom \workflows.up\ definitions in azure.yaml currently use the legacy sequential workflow runner, bypassing the new exegraph execution engine (PR #7776). This means \zd up\ has two different execution models depending on whether custom workflows are configured.

## Proposal
Extend the exegraph engine to model custom workflow steps as graph nodes, so \zd up\ uses a single unified execution model regardless of workflow configuration. This would enable custom workflows to benefit from the same parallelism, progress tracking, and error handling as the default path.

## References
- PR #7776 review thread: https://github.com/Azure/azure-dev/pull/7776#discussion_r3125966917
- Reviewer: @weikanglim