Narrow is_a? checks joined by || to their union type

[apiology]

This PR was written by Claude Code on behalf of @apiology.

Problem: solargraph typecheck --level strong reports a call as accepting the full, unnarrowed union of a variable's declared type, even inside an if branch that has already ruled out some of those types via ||-joined is_a? checks.

class Repro
  # @param e [Array<Symbol>, String]
  def eval_function_call(e); end

  # @param e [Array<Symbol>, String, Integer]
  def eval(e)
    if e.is_a?(Array) || e.is_a?(String)
      eval_function_call(e)  # reports e as Array<Symbol>, String, Integer
    end
  end
end

This produces a false Wrong argument type problem for any code relying on this common idiom to narrow a variable before use.

Solution: When both sides of an || are plain is_a? checks on the same variable, narrow that variable to the union of the two checked types inside the if branch; any other shape - different variables, or a side that isn't a plain is_a? call - is left unnarrowed, same as before.

Known limitation: a chain of three or more is_a? checks (a.is_a?(X) || a.is_a?(Y) || a.is_a?(Z)) still isn't narrowed, since the AST nests as (X || Y) || Z and the outer left side is an :or node rather than a :send. This degrades to the pre-existing no-narrowing behavior rather than narrowing incorrectly.