XLS(X) allows to store in cells date/datetimes values as date/datetime objects. Meaning, instead of having a string with the date in some format, the cell value can hold an actual date/datetime object, which are automatically converted into the `datetime.date`/`datetime.datetime` when the xls(x) file is parsed in python.

When an xls(X) file contains at the same time date values under date/datetime objects and under strings with the user format `%d/%m/%Y` (rather than the server format `%Y-%m-%d`),
the import was failing with the error
`time data '06/30/2025' does not match format '%Y-%m-%d'`

This error normally happens when the user tries to do an import with different kind of date formats under strings in the same file e.g. `06/30/2025` and `2025-07-01`
and this is understandable that Odoo doesn't know what to do in such a case.
But, if you stick to the same format, either only `%d/%m/%Y` either only '%Y-%m-%d'`, Odoo supports it.

However the case here is trickier: it's when the file contains at the same time dates under a string format, e.g. `06/30/2025` and under date objects, e.g. `datetime.date(2025, 6, 30)`. Which can happen quite easily, as Google Spreadsheet for instance tends to automatically convert the cells holding a date value into date object. And it's then easy to have a file containing both date objects and strings for date, which looks visually the same in the Google Spreadsheet interface.

In addition Google Spreadsheet tends to convert automatically only dates below the 12 of the month because of the american format. e.g.
if you set `01/06/2025`, it gets converted into a datetime object if you set `13/06/2025`, it doesn't get converted into a datetime object, the value stays as a string.

The goal of this revision is to support to have the possibility of having date values under date objects and string in a user format

The problem lied in the fact, during the xls parsing, date objects were converted into strings using the server date format. And then you could finish with data containing both the user format and the server format.

The idea is to no longer automatically convert date objects and to support having date objects in the import parsing. And when trying to guess the date/datetime format of the file, date objects are simply ignored, as they do not need to be parsed.