Improve Deployments and Size quota specs for clarity and consistency

What does this MR do and why?

Improves the Gitlab::Ci::Pipeline::Quota::Deployments and Gitlab::Ci::Pipeline::Quota::Size specs for clarity and consistency.

Note: The flakiness was already addressed in !223864 (merged). This MR focuses on:

• Using plan_limits.destroy! instead of stubbing actual_plan to simulate "limit does not exist" — this is explicit about what "no limit" means and avoids relying on obscure Plan.new.actual_limits behavior (per @engwan's suggestion)
• Using let_it_be_with_refind(:plan_limits) — necessary because destroy! freezes the AR object, and refind returns a fresh object from DB each time (unlike reload which mutates the same frozen object)
• Ensuring consistency between both sister specs
• Removing deployments_spec.rb from rspec_order_todo.yml since it can now run in random order

Changes in deployments_spec.rb:

• Changed let_it_be_with_refind(:namespace) to let_it_be(:namespace) (simpler, no need for refind)
• Added let_it_be_with_refind(:plan_limits) that creates a PlanLimits for the default plan
• Replaced allow(namespace).to receive(:actual_plan).and_return(Plan.new) with plan_limits.destroy!
• Removed from rspec_order_todo.yml

Changes in size_spec.rb:

• Changed let_it_be(:plan_limits, reload: true) to let_it_be_with_refind(:plan_limits)
• Replaced allow(namespace).to receive(:actual_plan) { create(:default_plan) } with plan_limits.destroy!

References

• Related incident: https://app.incident.io/gitlab/incidents/7509
• Failing job: https://gitlab.com/gitlab-org/security/gitlab/-/jobs/13154657369
• Related MR that fixed the flakiness: !223864 (merged)
• @engwan's suggestion to use plan_limits.destroy!: !223864 (comment 3092145946)

Screenshots or screen recordings

Not applicable - test improvement only

How to set up and validate locally

1. Run both specs multiple times with random order:

   for i in {1..10}; do bundle exec rspec spec/lib/gitlab/ci/pipeline/quota/deployments_spec.rb spec/lib/gitlab/ci/pipeline/quota/size_spec.rb --order random; done

2. All runs should pass without failures.

MR acceptance checklist

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