This fixes an issue where calendar event data could be grouped incorrectly when reports or views used date-based grouping such as by month. Business users will see more reliable calendar summaries and grouped results, with added tests to help prevent the problem from returning.

Commit b0f1086 forgot that `read_group` supports
aggregation functions (e.g. "start:month") and that
`groupby` could be a single field or a list of fields.

Task 2238100



--
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

This update prevents errors when Belgian payroll fleet CO2 fees are calculated for multiple vehicles or records at once. It improves reliability for payroll processing involving company cars without changing business rules.

Self can be multi so it bring error when we call _get_co2_fee

Opening the Calendar / Online Appointments menu no longer triggers an unexpected user warning. This lets users view and organize appointment-related calendar events normally, including grouping events by online appointment.

Click on menu Calendar/Online Appointments leads to a user warning

Task 2238100