Fix instance variable access

[shashankmehra]

After a recent change instance variables were not properly accessible or shared between steps in RSpec steps. This caused issues when trying to:

• Set instance variables in one step and access them in subsequent steps
• Use before blocks that set instance variables
• Share state between different steps in the same test suite

This PR implements several key changes to fix instance variable access:

• Modified StepExampleRunner to copy all instance variables from the context example during initialization
  • This ensures that instance variables set in the context are available to all steps
• Prefixed internal runner variables with underscore (@_context_example, @_running_example, etc.)
  • This prevents conflicts between internal step runner state and user-defined instance variables