Add Expression.asBoolean()

[cherylEnkidu]

Compared with the Android SDK implementation (reference: the linked commit), Android provides static functions such as Expression.rawFunction and BooleanExpression.rawFunction to create raw expressions. In the iOS SDK, both Expression and BooleanExpression are protocols, and we prefer not to introduce additional static factory methods. Instead, both use cases are covered by the FunctionExpression() constructor, which provides equivalent functionality without adding static APIs.

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	1 Warning
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[cherylEnkidu]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a significant and beneficial refactoring by changing BooleanExpression from a class to a protocol, which is more idiomatic in Swift. The new Expression.asBoolean() method provides a clean and type-safe way to perform conversions. The changes are well-implemented and include corresponding test updates. I have a couple of suggestions to enhance the code further, mainly regarding code style and ensuring the public API remains consistent.

[gemini-code-assist]

For better readability and to follow Swift idioms, consider using a switch statement here instead of a chain of if-let statements. A switch statement makes the different cases clearer and is often more efficient for type casting.

  func asBoolean() -> BooleanExpression {
    switch self {
    case let boolExpr as BooleanExpression:
      return boolExpr
    case let constant as Constant:
      return BooleanConstant(constant)
    case let field as Field:
      return BooleanField(field)
    case let funcExpr as FunctionExpression:
      return BooleanFunctionExpression(funcExpr)
    default:
      // This should be unreachable if all expression types are handled.
      fatalError(
        "Unknown expression type \(Swift.type(of: self)) cannot be converted to BooleanExpression"
      )
    }
  }