ARM request rate limiter for parallel provisioning (429 backoff)

## Context
With parallel layer provisioning, multiple ARM deployments run simultaneously. In shared subscriptions with other users also provisioning, this can trigger ARM's token-bucket throttling (HTTP 429). Without coordinated backoff, parallel layers may enter a retry storm where all layers compete for the same refilling bucket.

## Proposal
Add a cross-layer rate limiter that:
1. Monitors ARM responses for 429 status codes
2. When detected, applies exponential backoff across all in-flight layer provisioning
3. Coordinates retry timing to avoid thundering herd

This could be implemented as a shared `http.RoundTripper` wrapper that all provision layers use.

## References
- PR #7776 review thread: https://github.com/Azure/azure-dev/pull/7776#discussion_r3136349938
- Reviewer: @tg-msft