## Summary
This PR optimizes memory consumption and execution time when assigning a **Chart of Accounts** or **Fiscal Localization**. By moving filtering logic to the database and preventing expensive field prefetching, we've achieved a **60% reduction in peak memory** and cut execution time by more than half on large datasets.

## The Problem
Assigning a chart template was hitting memory limits on databases with a high volume of products (e.g., 2M+). Two main bottlenecks were identified:
* **Inefficient filtering**: Loading all `product.template` <-> `tax` relations into the cache and filtering in-memory using python instead of using SQL.
* **Excessive prefetching**: Accessing `product.product` fields (like `write_date`) inside the compute function triggered a cache miss that prefetched all product fields, consuming significant memory.

## Improvements
* **SQL Filtering:** Pushed the `product_template` filtration logic to the SQL layer to reduce the amount of data loaded into the memory.
* **Prefetching Prevention:** Optimized the compute logic to avoid triggering unnecessary field prefetching on `product.product`.

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## Benchmarks
*Tested using `memray` on a customer database with ~2 million products.*

| Scenario | Duration | Peak Memory | Total Allocations |
| :--- | :--- | :--- | :--- |
| **Baseline (Before)** | 10:23.4 | 3.6 GB | 9,954,480 |
| **Optimized Prefetching Only** | 10:21.0 | 2.3 GB | 9,292,271 |
| **SQL Filtering Only** | 06:30.2 | 3.0 GB | 8,865,050 |
| **Combined (Final Result)** | **04:59.6** | **1.4 GB** | **8,213,374** |

### Key Results:
* **Memory Saved:** ~2.2 GB (61% reduction)
* **Time Saved:** ~5.5 minutes (52% faster)

OPW-6070666

Forward-Port-Of: odoo/odoo#259304