fix: should not skip auto now fields but assign with specified value

[mattwang44]

Describe the change
Fix #519, which is a regression introduced in #507.

• The auto now fields should not be skipped in _skip_field(). Instead, assign the specified value to the resulting instance.
• Add test cases (modified from those provided by @hoefling in #519).

PR Checklist

• Change is covered with tests
• CHANGELOG.md is updated if needed

[Copilot]

PR Overview

This PR fixes a regression that prevented assigning specified values to auto now fields by modifying the field handling in Baker. Key changes include:

• Removing the automatic skipping of auto now fields in _skip_field.
• Updating _handle_auto_now to perform a forced update via the model’s manager and then manually assign the specified values.
• Adding new test cases for auto now fields in both recipes and baker tests, and updating the CHANGELOG accordingly.

Reviewed Changes

File	Description
tests/test_recipes.py	Added TestAutoNowFields with two test cases using different value generators.
model_bakery/baker.py	Removed skip for auto now fields in _skip_field and revised _handle_auto_now to update and assign fields.
tests/test_baker.py	Extended tests to assert that auto now fields remain unchanged after refresh.
CHANGELOG.md	Documented the fix for the regression regarding auto now field updates.

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

[amureki]

Good change!
This indeed feels like a better solution, thanks for the fix @mattwang44