fix: check account exists before calling BillingService.delete_account

[ifer47]

Summary

In delete_account_task, the account existence check was placed after the billing service deletion call. If the account doesn't exist, BillingService.delete_account() would have already been executed (which may make irreversible external changes) before the task discovers the account is missing and returns early.

Before

account = session.scalar(...)
try:
    if dify_config.BILLING_ENABLED:
        BillingService.delete_account(account_id)  # Runs even if account is None!
except Exception:
    raise

if not account:
    return  # Too late — billing deletion already happened

After

account = session.scalar(...)
if not account:
    return  # Bail out before making any external changes

try:
    if dify_config.BILLING_ENABLED:
        BillingService.delete_account(account_id)
except Exception:
    raise

Test plan

• Logic is straightforward — the check is simply moved above the billing call
• Verify that BillingService.delete_account with a non-existent account ID would have caused side effects (the current code path would have hit this in production for any stale Celery task retry)

🤖 Generated with Claude Code Best

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	48.59%	48.59%	0.00%
Strict coverage	48.10%	48.10%	0.00%
Typed symbols	27,995	27,995	0
Untyped symbols	29,922	29,922	0
Modules	2892	2892	0