Add assert_unsuccessful_code function

[Chemaclass]

📚 Description

Closes: #505 by @jrfnl

Add assert_unsuccessful_code assertion function to verify that a command or function exits with a non-zero exit code. This complements the existing assert_successful_code function, providing a semantic way to assert failures without needing to specify an exact exit code.

🔖 Changes

• Add assert_unsuccessful_code function
  • Asserts that the exit code is non-zero (any value other than 0)
  • Follows the same pattern as assert_successful_code
  • Accepts optional callable parameter, defaults to $? if not provided
• Add tests and documentation for it

✅ To-do list

• I updated the CHANGELOG.md to reflect the new feature or fix
• I updated the documentation to reflect the changes

[jrfnl]

Awesome! Thank you @Chemaclass !