Optimize factory usage for instance_clusterable_presenter_spec

What does this MR do and why?

This MR replaces create with build_stubbed in ee/spec/presenters/ee/instance_clusterable_presenter_spec.rb to optimize the 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 ee/spec/presenters/ee/instance_clusterable_presenter_spec.rb.

Factory Usage Optimization Results

Before optimization:

• Factory time: 2.81s (14.5% of total time)

After optimization:

• Factory time: 0.05s (0.32% of total time)

🚀 Improvement:

• Factory time reduced by: 2.76s
• Factory time percentage reduced by: 14.18%
• Overall factory usage improvement: 98.2%

Factory Doctor Output

[TEST PROF INFO] FactoryDoctor enabled (event: "sql.active_record", threshold: 0.01)
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}
==> Using precompiled Gitaly binaries from cache
==> Go projects set up in 0.754 seconds...

Test environment set up in 2.115037999988999 seconds
..[TEST PROF INFO] FactoryDoctor report

Total (potentially) bad examples: 2
Total wasted time: 00:02.590

InstanceClusterablePresenter (./ee/spec/presenters/ee/instance_clusterable_presenter_spec.rb:5) (40 records created, 00:02.590)
  is expected to eq "/admin/clusters/57/environments" (./ee/spec/presenters/ee/instance_clusterable_presenter_spec.rb:23) – 20 records created, 00:01.953
  is expected to be nil (./ee/spec/presenters/ee/instance_clusterable_presenter_spec.rb:29) – 20 records created, 00:00.637


Finished in 14.35 seconds (files took 15.44 seconds to load)
2 examples, 0 failures

Randomized with seed 50628

[TEST PROF INFO] Time spent in factories: 00:02.601 (13.93% of total time)

Factory time after update

[TEST PROF INFO] Time spent in factories: 00:00.050 (0.32% of total time)

References

Relates to: #378910.

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