fix: change Debian default MPM from worker to event

[damacus]

Summary

Modern Debian installations (10+) default to the event MPM, not worker. This PR aligns the cookbook with the actual OS defaults.

Changes

• Updated default_mpm in libraries/helpers.rb to return 'event' for Debian (same as Ubuntu and Linux Mint)
• Updated corresponding test in spec/libraries/mpm_support_spec.rb

Testing

• Unit tests updated to expect event for Debian
• CI will verify the change works across all platforms

Fixes #821

[github-actions]

Slowest examples

Top 10 slowest examples (3.16 seconds, 15.54% of total time)

Example	Description	Time in seconds
spec/resources/mod_spec.rb:166	apache2_install mod_ldap outputs template correctly	0.50868
spec/resources/mod_auth_cas_spec.rb:27	apache2_mod_auth_cas ubuntu is expected to enable apache2_module "auth_cas"	0.46071
spec/libraries/pid_file_spec.rb:15	#apache_pid_file with fedora family is expected to write log "/var/run/httpd/httpd.pid"	0.36047
spec/libraries/package_helpers_spec.rb:97	#default_apache_pkg with Debian is expected to write log "apache2"	0.34741
spec/libraries/log_dir_spec.rb:35	#log_dir with arch is expected to write log "/var/log/httpd"	0.28857
spec/libraries/devel_package_spec.rb:26	#apache_devel_package suse is expected to write log "apache2-devel"	0.2642
spec/resources/conf_spec.rb:34	apache2_install install an apache2_module with default properties Creates the ports template	0.24859
spec/resources/mod_auth_cas_spec.rb:55	apache2_mod_auth_cas ubuntu is expected to render file "/etc/apache2/mods-available/auth_cas.conf"	0.23474
spec/resources/default_site_spec.rb:20	apache2_install Enable the default site Creates the default site template	0.22815
spec/resources/mod_auth_cas_spec.rb:55	apache2_mod_auth_cas ubuntu is expected to render file "/etc/apache2/mods-available/auth_cas.conf"	0.22523