Fix ChatGPT subscription model in byteask.ps1

[h30s]

Fixes #3

Summary of Changes

• Omit managed model in subscription config (byteask.ps1):
  • In Invoke-ByokSubscription, leave $model empty rather than defaulting to 'gpt-5.4' (matching the behavior in byteask:1257-1259). This allows byteask-engine to resolve a valid default model against OpenAI's live ChatGPT catalog instead of failing with Model metadata for 'gpt-5.4' not found.
  • In Write-SubscriptionConfig, emit model = "$model" only when $model is non-empty, preventing invalid model = "" lines.
• Preserve catalog when switching back (byteask.ps1):
  • In Invoke-ByokOff, fall back to $BYTEASK_HOME/models-catalog.json when model_catalog_json was dropped by a prior subscription session, preventing the catalog from being permanently dropped.
• Regression test (tests/test_subscription.ps1):
  • Verifies Write-SubscriptionConfig omits the model = line when empty.
  • Verifies custom models are preserved when passed.
  • Verifies Invoke-ByokOff restores model_catalog_json.