Currently, an error occurs when clicking the `Send to eTIMS` button on vendor bills.

Steps to Reproduce:
 - Install `l10n_ke_edi_oscu` module without demo data.
 - Create a New company with `Kenya` as the Country and switch to it.
 - Go to Vendors > Bills, create a new bill, and add an invoice line without tax.
 - Confirm it, then click `Send to eTIMS`.

Traceback:
```py
 File "/home/odoo/src/enterprise/19.0/l10n_ke_edi_oscu/models/account_move.py", line 551, in action_l10n_ke_oscu_confirm_vendor_bill
    content = move._l10n_ke_oscu_json_from_move()
  File "/home/odoo/src/enterprise/19.0/l10n_ke_edi_oscu/models/account_move.py", line 199, in _l10n_ke_oscu_json_from_move
    line_items = self._l10n_ke_oscu_get_json_from_lines(tax_details)
  File "/home/odoo/src/enterprise/19.0/l10n_ke_edi_oscu/models/account_move.py", line 250, in _l10n_ke_oscu_get_json_from_lines
    tax, line_tax_details = next(
StopIteration: null
```

This error occurs because when a new company is created, the `eTIMS Server Mode` in Settings is empty. As a result, `l10n_ke_oscu_is_active` field becomes `False`, and at [1] the `l10n_ke_validation_message` field will also be `False`, causing the flow to be skipped. Therefore, no error is raised on the frontend side. The field `l10n_ke_oscu_is_active` is set to `True` only when `eTIMS Server Mode` is set to `Demo`.

Here we raise a warning when the `eTIMS` configuration is not set up correctly.

[1]: https://github.com/odoo/enterprise/blob/7fb7b3168039f00b6d815202bc19ac35aa1d9b5e/l10n_ke_edi_oscu/models/account_move.py#L91-L93

sentry-7083978544

Forward-Port-Of: odoo/enterprise#101319