CHANNEL_SERVICES held frozen references to service class objects, captured
when SendReplyJob was first autoloaded. In test env (cache_classes=false),
Zeitwerk reloads triggered between specs (notably by request specs)
replaced the constants with new class objects, leaving the hash pointing
to the stale ones. RSpec stubs applied to the current constant were then
bypassed when the job called service_class.new(...) through the stale
reference, causing flaky CI failures in spec/jobs/send_reply_job_spec.rb
when sharded together with V2::ReportBuilder + Captain::Preferences specs.
Storing class names as strings and resolving via constantize per call
fixes this and is the standard Rails autoload-safe pattern.
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