[WIP][18.0][MIG] sale_custom_rounding: Migration to 18.0

[Anxo82]

@HaraldPanten

• Replaced the stored compute on tax_calculation_rounding_method with an @api.onchange.
  In Odoo 18 the field is defined in core as a company-related field, so the stored compute approach no longer worked reliably (it could be computed too early and stored with a wrong value).

• Removed the override of _compute_tax_totals_json.
  In Odoo 18 the tax rounding is handled by the new tax engine and no longer depends on the round context flag, so the custom context-based logic was no longer applicable.

• Simplified the override of _compute_amount on sale.order.line. The method now simply extends the @api.depends to trigger recomputation when the rounding method changes and delegates the computation entirely to the core logic.

• [T-9321]

[HaraldPanten]

/ocabot migration sale_custom_rounding

[Jaimermaccione]

@Anxo82 Module functionally tested and reviewed in a local environment.

Tests Performed

• Configured a tax rounding method at contact level → correctly inherited in the Sales Order.
• Configured a different tax rounding method at invoicing address level → invoicing address setting correctly overrides the main contact.
• Generated invoices from multiple Sales Orders with:
  • Different rounding methods
  • Same customer with different rounding configurations

Results

As described in the README, the tax rounding method is used as a grouping key during invoice creation. The system correctly generates one invoice per tax rounding method.

Conclusion

The README accurately reflects the module behavior.
Functionality works as expected.

LGTM. Thank you!