Enhancement: Add validation to prevent --groupby and --deferred-fields conflict

[bosd]

Related Issue

This is related to #185 which documents the bug. This issue is for the enhancement to add validation.

Proposal

Add validation logic to detect when the same field appears in both --groupby and --deferred-fields arguments.

Suggested Implementation

In the argument parsing or pre-flight check:

if groupby_field and groupby_field in deferred_fields:
    raise ValueError(
        f"Field '{groupby_field}' cannot be used in both --groupby and --deferred-fields. "
        f"Remove --groupby or remove the field from --deferred-fields."
    )

Alternatives

1. Auto-fix: Automatically remove the field from groupby when it's deferred (with a warning)
2. Documentation: At minimum, document this incompatibility

Context

This was discovered during an Odoo 12 → 18 migration where res.partner parent/child relationships were silently broken due to this flag combination in the import script.