feat(billing): enable Stripe automatic tax on checkout sessions (#13058)

## Summary

Enables Stripe Tax automatic calculation on both the subscription
upgrade and credit top-up checkout sessions.

## Changes

**File:** `autogpt_platform/backend/backend/data/credit.py`

Added three parameters to both `stripe.checkout.Session.create` calls:

```python
automatic_tax={enabled: True},
billing_address_collection=auto,
customer_update={address: auto},
```

**Affected functions:**
- `create_subscription_checkout()` — "Upgrade to Max" / plan upgrade
flow
- `top_up_intent()` — "Add credits" top-up flow

## Why each parameter

| Parameter | Reason |
|---|---|
| `automatic_tax` | Activates Stripe Tax calculation at checkout |
| `billing_address_collection="auto"` | Collects country + postcode
(minimum required for tax rate lookup) |
| `customer_update={"address": "auto"}` | Saves address back to Stripe
Customer so repeat checkouts don't re-prompt |

## Prerequisites (Stripe Dashboard — before deploying)

1. Enable Stripe Tax: Dashboard → Tax → Enable
2. Set a default tax code on each product (e.g. `txcd_10000001` for
SaaS)
3. Set default tax behavior (`exclusive` or `inclusive`)
4. Register at least one tax nexus jurisdiction

## Testing

- Checkout in test mode with a US address (e.g. California) → should
show sales tax line item
- Checkout with a UK address → should show VAT
- Returning customer with saved address → should not be re-prompted for
address

Co-authored-by: Toran Bruce Richards <22963551+Torantulino@users.noreply.github.com>