Add null guards to prevent fatal errors during pp-hosts plugin updates

[ilicfilip]

When pp-hosts (or other host plugins bundling Progress Planner) is updated, hooks registered by the old plugin version can fire during the update process. At that point, dependencies may not be available because files are being extracted, causing fatal errors like:

• Call to a member function get_post_types_names() on null
• Class "Progress_Planner\Activities\Maintenance" not found

This PR fixes that by adding checks and making early exits:

• Add null check for get_activities__content_helpers() in Content class should_skip_saving() method
• Add null checks in Content_Scan class get_total_pages() and update_stats() methods
• Add class_exists() check in Maintenance class before instantiating Activities_Maintenance

[github-actions]

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[github-actions]

✅ Code Coverage Report

Metric	Value
Total Coverage	31.61% 📉
Base Coverage	31.63%
Difference	📉 -0.02%

⚠️ Coverage below recommended 40% threshold

📊 File-level Coverage Changes (3 files)

📉 Coverage Decreased

Class	Before	After	Change
Progress_Planner\Actions\Maintenance	33.33%	30.77%	-2.56%
Progress_Planner\Actions\Content	91.92%	91.18%	-0.74%
Progress_Planner\Actions\Content_Scan	3.57%	2.94%	-0.63%

ℹ️ About this report

• All tests run in a single job with Xdebug coverage
• Security tests excluded from coverage to prevent output issues
• Coverage calculated from line coverage percentages