fix(test): backdate UserBalance.updatedAt in test_block_credit_reset (#12236)

## Root cause

The test constructs \`month3\` using \`datetime.now().replace(month=3,
day=1)\` — hardcoded to **March of the real current year**. When
\`update(balance=400)\` runs, Prisma auto-sets \`updatedAt\` to the
**real wall-clock time**.

The refill guard in \`BetaUserCredit.get_credits\` is:
\`\`\`python
if (snapshot_time.year, snapshot_time.month) == (cur_time.year,
cur_time.month):
    return balance  # same month → skip refill
\`\`\`

This means the test only fails when run **during the real month of
March**, because the mocked \`month3\` and the real \`updatedAt\` both
land in March:

| Test runs in | \`snapshot_time\` (real \`updatedAt\`) | \`cur_time\`
(mocked month3) | Same? | Result |
|---|---|---|---|---|
| January 2026 | \`(2026, 1)\` | \`(2026, 3)\` | ❌ | refill triggers ✅ |
| February 2026 | \`(2026, 2)\` | \`(2026, 3)\` | ❌ | refill triggers ✅
|
| **March 2026** | **\`(2026, 3)\`** | **\`(2026, 3)\`** | **✅** |
**skips refill ❌** |
| April 2026 | \`(2026, 4)\` | \`(2026, 3)\` | ❌ | refill triggers ✅ |

It would silently pass again in April, then fail again next March 2027.

## Fix

Explicitly pass \`updatedAt=month2\` when updating the balance to 400,
so the month2→month3 transition is correctly detected regardless of when
the test actually runs. This matches the existing pattern used earlier
in the same test for the month1 setup.

## Test plan
- [ ] \`pytest backend/data/credit_test.py::test_block_credit_reset\`
passes
- [ ] No other credit tests broken

autogpt_platform/backend/backend/data/credit_test.py

@@ -178,9 +178,13 @@ async def test_block_credit_reset(server: SpinTestServer):
         assert month2_balance == 1100  # Balance persists, no reset
 
         # Now test the refill behavior when balance is low
-        # Set balance below refill threshold
+        # Set balance below refill threshold and backdate updatedAt to month2 so
+        # the month3 refill check sees a different (month2 → month3) transition.
+        # Without the explicit updatedAt, Prisma sets it to real-world NOW which
+        # may share the same calendar month as the mocked month3, suppressing refill.
         await UserBalance.prisma().update(
-            where={"userId": DEFAULT_USER_ID}, data={"balance": 400}
+            where={"userId": DEFAULT_USER_ID},
+            data={"balance": 400, "updatedAt": month2},
         )
 
         # Create a month 2 transaction to update the last transaction time