Per-service previewer output buffering for concurrent deploy

## Context
During concurrent service deployments (introduced in PR #7776), multiple services write progress to the shared console previewer simultaneously. The output is concurrency-safe (no crashes or corruption), but lines from different services interleave, making it hard to follow individual service progress.

## Proposal
Buffer console output per-service and display one service's output at a time, or use separate rendering regions per service. The deploy progress table already handles per-service status cleanly; this is about the verbose log output underneath.

## References
- PR #7776 review thread: https://github.com/Azure/azure-dev/pull/7776#discussion_r3125952137
- Reviewer: @weikanglim