Fix typo and add instance variable test coverage for flow sensitive typing

[Copilot]

Code review of the ivar support implementation identified a typo and missing test coverage for the core feature.

Changes

• Fixed typo in comment (line 63 of flow_sensitive_typing.rb): "so't provide" → "so provide"
• Added test for instance variable type refinement: Validates .is_a?() correctly narrows instance variable types in conditionals

Example

The new test validates this behavior:

class Example
  # @param value [ReproBase]
  def initialize(value)
    @value = value
  end

  def check
    if @value.is_a?(Repro)
      @value  # Now typed as Repro
    else
      @value  # Now typed as ReproBase (narrowed from union)
    end
  end
end

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