Replace machinist with FactoryBot for test data setup

[philippthun]

Per ADR 0015. machinist 1.0.6 has had no upstream activity since 2013 and machinist 2.0 is not a viable upgrade (no Sequel adapter, no Sham, non-persisting make). FactoryBot is actively maintained, has trivial Sequel integration, and provides equivalents for every pattern used by the existing blueprints.

Convert ~11k call sites from Model.make / Model.make_unsaved to create(:foo) / build(:foo), replace ~130 blueprints with factory definitions under spec/support/factory_definitions/, and remove the machinist gem along with spec/support/fakes/blueprints.rb and spec/support/machinist_monkey_patch.rb.

Key technical decisions (see ADR 0015):

• Global to_create { |i| i.save; i.refresh } in spec/support/factories.rb. This matches machinist's Sequel adapter which both saved and refreshed; the refresh prevents stale-cache bugs whenever a factory's after-create hook mutates associations.
• Sham is preserved as a thin shim (spec/support/sham_shim.rb) that delegates Sham.<name> to FactoryBot.generate(:sham_<name>) sequences defined in spec/support/factories.rb. The shim mirrors the original Sham.define block 1:1 so existing call sites need no edits.
• Dynamic class -> factory-name conversion via klass.name.demodulize.underscore.to_sym is used in matchers and shared examples, so generic helpers continue to look up the right factory when given any model class.
• Named blueprints become traits: Foo.blueprint(:bar) turns into a trait :bar on the :foo factory; Foo.make(:bar, x: 1) becomes create(:foo, :bar, x: 1).
• build replaces make_unsaved for the (rare) cases that wanted an unsaved instance.
• :droplet_model only auto-sets itself as the app's current droplet when no app: override is supplied (set_as_current_droplet { app == :unset }), matching machinist's blueprint where the default app block only ran in that case. Specs that previously relied on the auto-set side effect when passing app: explicitly call app.update(droplet:) just as the pre-migration versions did.
• :revision_sidecar_process_type_model builds its parent with the :no_process_types trait so FactoryBot.lint does not collide with the parent's after-create web row on the (revision_sidecar_guid, type) unique constraint.

• I have reviewed the contributing guide

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• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests

[johha]

Whats the reason for pinning the version?

[philippthun]

We also pin other test gems (rspec, rubocop). Dependabot will update it accordingly.

[johha]

Remove comment?

[philippthun]

I kept a single line commit.