fix(api): preserve managed-home provider preference

[haasonsaas]

Summary

Platform persistent-computer requests send workspace_mode=persistent and provider_preference=cloudflare to POST /v1/homes/{home_id}/sandboxes. The managed-home wrapper replaced that preference with any, so the Cloudflare worker could not claim the job after sandboxwich#334 tightened provider matching.

This change carries the request's explicit provider preference into the managed-home provision spec. An omitted preference still defaults to any.

Regression

The HTTP contract test registers Kubernetes and Cloudflare workers, creates a persistent managed-home sandbox with provider_preference=cloudflare, and verifies:

• Kubernetes receives no lease.
• Cloudflare claims the provision job.
• The queued provisionSpec.provider_preference remains cloudflare.

Tests

• cargo test -p sandboxwich-api --test http_contract homes::
• cargo test -p sandboxwich-api provider_preference
• cargo test -p sandboxwich-api --test http_contract sandboxes::
• cargo fmt --all -- --check