Optimize factory usage in subscription_entity_spec

What does this MR do and why?

This MR replaces create with build_stubbed in spec/serializers/jira_connect/subscription_entity_spec.rb to optimize factory usage as recommended in our handbook. This spec is part of the list .rubocop_todo/rspec/factory_bot/avoid_create.yml. It was verified that database persistence is not needed using the factory doctor command FDOC=1 bin/rspec spec/serializers/jira_connect/subscription_entity_spec.rb.

Factory Usage Optimization Results

Before optimization:

• Factory time: 0.375s (2.75% of total time)

After optimization:

• Factory time: 0.024s (0.19% of total time)

Improvement:

• Factory time reduced by: 0.351s
• Factory time percentage reduced by: 2.56%
• Overall factory usage improvement: 93.6%

Factory Doctor Output

[TEST PROF INFO] FactoryDoctor report

Total (potentially) bad examples: 1
Total wasted time: 00:00.320

JiraConnect::SubscriptionEntity (./spec/serializers/jira_connect/subscription_entity_spec.rb:5) (7 records created, 00:00.320)
  contains all necessary elements of the subscription (./spec/serializers/jira_connect/subscription_entity_spec.rb:12) – 7 records created, 00:00.320

This MR also adds the missing feature_category to this spec.

References

Relates to: #378910

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