Evaluate fixture definitions in adapter execution context

[ngan]

Summary

This updates fixture definition evaluation so fixture blocks run on the real framework execution context (RSpec example instance / Minitest test instance) while still routing expose(...) back to the current FixtureKit::Definition.

Changes

• Pass execution context from adapters into definition evaluation:
  • lib/fixture_kit/adapters/rspec_adapter.rb
  • lib/fixture_kit/adapters/minitest_adapter.rb
  • lib/fixture_kit/cache.rb
• Change FixtureKit::Definition#evaluate to require a context and execute the definition on that context.
• Dynamically prepend a small mixin to the context singleton class to provide expose(...) delegation to the current definition.
• Add regression coverage for helper methods inside fixture blocks (travel_to in RSpec integration).
• Update unit tests for Definition#evaluate(context).

Why

Fixture blocks could not call instance helper methods from the framework test context (e.g. travel_to from included RSpec helpers), because definitions previously executed with self == FixtureKit::Definition.

Validation

• bundle exec rspec --fail-fast spec/unit/definition_spec.rb spec/unit/rspec_adapter_spec.rb spec/unit/minitest_adapter_spec.rb spec/unit/fixture_cache_spec.rb spec/integration/dummy_app_spec.rb
• FIXTURE_KIT_INTEGRATION_FRAMEWORK=minitest bundle exec rspec --fail-fast spec/integration/dummy_app_spec.rb