Make __undefined__ processing recursive

[han4wluc]

Problem

Previously, undefined only worked for top-level values in test assertions. Nested structures like { prop: __undefined__ } would not convert the string to actual undefined values.

Solution

• Extract processUndefined() function to utils.js for reuse
• Apply recursive processing to all test case outputs, execution outputs, and assertions
• Remove duplicate undefined processing in mockResolver.js

Changes

• Updated test cases in examples/undefined.test.yaml to verify nested functionality
• All existing tests continue to pass (backward compatible)
• New capabilities:
  • Objects: { prop: __undefined__, nested: { value: __undefined__ } }
  • Arrays: [1, __undefined__, { nested: __undefined__ }]
  • Method executions: out: { result: __undefined__ }
  • Property assertions: value: __undefined__