### Issue before this commit:
When reverting a Guatemalan invoice (creating a credit note), the XML node FechaEmisionDocumentoOrigen incorrectly reports the EDI document's technical certification date instead of the original invoice's emission date. This causes the SAT to reject the document.

### Steps to reproduce the issue:
1. Download Accounting and l10n_gt
2. Revert an invoice (credit note) inserting a different date than the one of the invoice
3. See that FechaEmisionDocumentoOrigen report the date of the credit note instead of the one of the invoice

### Cause of the issue:
The _l10n_gt_edi_add_reference_values method extracted the date from original_document.datetime (the technical timestamp of when the XML was generated) rather than using the actual accounting date of the original invoice.

### Reason to introduce the fix:
SAT validation rules strictly require the reference date to match the exact commercial emission date of the original invoice. Fetching invoice_date directly ensures compliance, avoids timezone conversion errors, and prevents the XML from being rejected.
Source: https://www.lawinsider.com/es/contracts/dJXl4Vo79L2

<img width="730" height="205" alt="2026-07-17_10-19" src="https://github.com/user-attachments/assets/802e7bb3-fcf9-48db-b86f-227b494001b6" />

opw-6394409

**Description of the issue/feature this PR addresses:**

When adding a custom field (e.g., via Studio) to the Documents list view, clicking the cell directly does not trigger inline edit mode. The user has to first click a standard editable field (like "Owner") to put the row into edit mode before they can modify the custom field.

This occurs because we use a hardcoded whitelist (`editableColumns`) of standard fields allowed to trigger edit mode. Custom fields (`x_`) are missing from this static list.

This commit resolves the issue by dynamically injecting visible, non-readonly custom fields into the `editableColumns` whitelist. This allows user-created fields to be edited inline as expected.

**Steps to reproduce:**

- Documents > Studio > List view > Add any field that accepts user input (e.g. Text/char) > save/exit
- In the same Documents list view > select a row > click the cell belonging to the newly created field > observe that the row does not enter edit mode
- In the same Documents list view > select a row > click a standard editable cell, then click the cell belonging to our newly created field > observe that this then allows us to edit our field

**Current behavior before PR:**

- Custom fields do not trigger inline edit mode

**Desired behavior after PR is merged:**

- Custom fields trigger inline edit mode

opw-6378102