Optimize factory usage in suggestion_entity_spec

What does this MR do and why?

This MR replaces create with build_stubbed in spec/serializers/suggestion_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/suggestion_entity_spec.rb.

Factory Usage Optimization Results

Before optimization:

• Factory time: 6.049s (30.2% of total time)

After optimization:

• Factory time: 0.06s (0.43% of total time)

Improvement:

• Factory time reduced by: 5.989s
• Factory time percentage reduced by: 29.77%
• Overall factory usage improvement: 99.0%

Factory Doctor Output

[TEST PROF INFO] FactoryDoctor report

Total (potentially) bad examples: 2
Total wasted time: 00:01.889

SuggestionEntity (./spec/serializers/suggestion_entity_spec.rb:5) (18 records created, 00:01.889)
  returns the inapplicable reason (./spec/serializers/suggestion_entity_spec.rb:42) – 9 records created, 00:01.063
  returns appropriate message (./spec/serializers/suggestion_entity_spec.rb:50) – 9 records created, 00:00.825

This MR also adds the missing feature_category to this spec.

References

Relates to: #378910

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