feat: callable defaults with instance context and dependency support

[bharget]

Summary

Enhances callable defaults so they can access other attributes and depend on each other.

Changes

• Callable defaults (procs/lambdas) are evaluated with instance_exec, giving access to other attributes
• Defaults are processed in definition order, allowing callable-to-callable dependencies
• Explicit attributes are set first, so callables can reference them

Example

class GreetingService < ApplicationService
  argument :first_name, default: "John"
  argument :last_name, default: "Doe"
  argument :full_name, default: -> { "#{first_name} #{last_name}" }
  argument :greeting, default: -> { "Hello, #{full_name}!" }
end

GreetingService.call.greeting  #=> "Hello, John Doe!"
GreetingService.call(first_name: "Jane").greeting  #=> "Hello, Jane Doe!"

Tests

Added tests for:

• Callables accessing other attributes
• Callable-to-callable dependencies
• Explicit attributes accessible to callables
• Overriding callable defaults with explicit values