[codex] expose per-instance replica state on phala_app

[h4x3rotab]

Summary

This PR adds an additive per-instance view to phala_app without changing the backend API shape.

It keeps phala_app as the app-level lifecycle resource and exposes current app members through a computed instances attribute sourced from the existing app/CVM APIs.

What Changed

• add computed phala_app.instances
• each instance exposes:
  • id
  • app_id
  • name
  • vm_uuid
  • instance_id
  • status
  • region
  • instance_type
  • endpoint
  • created_at
• include created_at in the provider CVM response model
• extend the smoke fixture outputs with:
  • app_instances
  • app_instance_vm_uuids
  • app_instance_ids
• document the new computed state in the resource docs

Why

Issue #243 asks for primitives closer to GCP MIG / StatefulSet semantics for stateful replica sets.

Without backend API changes, the smallest honest Terraform slice is not a new lifecycle resource or fake ordinal model. It is exposing the real per-instance identity that already exists in the backend/API:

• app = set identity
• CVM = concrete instance identity
• vm_uuid = operator-safe per-instance handle
• instance_id = runtime/workload identity

This PR makes that shape available in Terraform state while preserving backward compatibility for existing phala_app users.

Intentionally Deferred

• no phala_app_instance resource yet
• no ordinals / slots
• no rollout policy semantics
• no lifecycle hooks / auto-heal semantics

Those need either a stronger backend contract or a separate provider design pass.

Validation

• go test ./internal/provider/...
• real-env smoke run against live Phala Cloud with replicas=2
  • verified phala_app.instances returned two concrete members
  • verified each member included distinct vm_uuid and instance_id
  • verified destroy cleaned up the smoke app

Additional Fixes Found During Smoke

The live smoke run surfaced two provider bugs that are fixed in this PR:

1. instances must be a Terraform types.List, not a plain Go slice, so plan-time unknown values are handled correctly.
2. Auto-placement should not backfill region into state when the user left region unset, otherwise apply can fail with an inconsistent result.

Closes #243