Refactor duplicated error class definitions in task-runner

[Copilot]

Three error classes in validators.js had identical implementations differing only in their name property.

Changes

• Introduced createErrorClass() factory function to generate error classes with consistent validation behavior
• Refactored StateError, UnknownDependencyError, and UnknownTaskError to use the factory
• Preserved DependencyCycleError as a standalone class due to its additional cyclePath property

Before:

StateError: class StateError extends Error {
  constructor(message) {
    ow(message, ow.string);
    super(message);
    this.name = 'StateError';
  }
},
// ... repeated for UnknownDependencyError, UnknownTaskError

After:

function createErrorClass(name) {
  const ErrorClass = class extends Error {
    constructor(message) {
      ow(message, ow.string);
      super(message);
      this.name = name;
    }
  };
  Object.defineProperty(ErrorClass, 'name', { value: name });
  return ErrorClass;
}

// Usage
StateError: createErrorClass('StateError'),
UnknownDependencyError: createErrorClass('UnknownDependencyError'),
UnknownTaskError: createErrorClass('UnknownTaskError'),

Reduces ~27 lines of duplicated code to a single reusable factory function.

Original prompt

Find and refactor duplicated code.

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