### Problem
Opening the "Invoices to be Issued" and "Invoiced Not Delivered" menus
caused MemoryErrors and 80+ second load times on large databases.

### Root Cause
`_read_group_for_accrual` loops over each group result to manually sum
non-stored computed fields. On each iteration it called `search()` with
a domain containing `('invoice_to_be_issued', '=', True)`, triggering
`_get_accrual_line_ids` once per group instead of once per request.

### Fix
Move the `search()` call outside the loop so matching records are fetched
once per request. The per-group partition is then resolved using
`search_fetch` + `grouped`, which handles the field prefetching and
grouping into recordsets in one call.

### Before / After
| Metric                        | Before  | After  |
|-------------------------------|---------|--------|
| Total request time            | 82s     | 8.5s   |
| `_get_accrual_line_ids` calls | 82      | 3      |
| MemoryError on large DBs      | ✓       | ✗      |

opw-5392606

Forward-Port-Of: odoo/enterprise#108102