fix: calculate last_grade_publish_date for all unreleased subsections

[asajjad2]

What are the relevant tickets?

#9459

Description (What does it do?)

The last_grade_publish_date field for each section in the progress API was showing the current datetime instead of the actual grade release date when there were no unreleased subsections with show_correctness values of 'past_due' or 'never_but_include_grade'.

The original code only initialized last_grade_publish_date to now() and updated only when:

1. show_correctness (Assessment result visibility) was in ['past_due', 'never_but_include_grade'] and
2. The due date was greater than the current last_grade_publish_date

This PR updates the code to initialize last_grade_publish_date from now() to None

Testing Steps

1. Create a course with subsections where:
   • All assignment due dates are in the past
   • Some have show_correctness=past_due or never_but_include_grade
2. As a student, view the Progress page on the Learning MFE
3. The Page should not show the warning Some assignment scores are not yet included in your total grade. These grades will be released by {current date}

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[arslanashraf7]

@asajjad2 could you add test(s) for this change?

[Anas12091101]

LGTM 👍

[Anas12091101]

This can be simplified to:

Suggested change

	if 'last_grade_publish_date_days' in expected:
	expected_date = (
	datetime.now(timezone.utc) +
	timedelta(days=expected['last_grade_publish_date_days'])
	)
	assert row['last_grade_publish_date'] is not None
	else:
	assert row['last_grade_publish_date'] is None
	assert (row['last_grade_publish_date'] is not None) == (
	'last_grade_publish_date_days' in expected
	)

[pdpinch]

@asajjad2 can you squash your commits and rebase?