fix: allow coercion of string booleans with trailing whitespace

[AnslemHack]

Description:

This PR addresses issue #3091 by improving the coercion behavior of Joi.boolean() for string inputs with trailing whitespace.

Problem
Currently, Joi.boolean().validate('true ') fails with a validation error, even though Joi.number().validate('123 ') successfully coerces the string to a number. This inconsistency creates friction when validating user input from forms, query params, or loosely formatted sources where trailing whitespace is common.

Solution
This update trims string inputs before coercion in the boolean type, aligning its behavior with number. It ensures that:

js
Joi.boolean().validate('true ') // → { value: true }
Joi.boolean().validate('false ') // → { value: false }
Tests
Unit tests have been added to cover:

"true " → true

"false " → false

" true " → true

"false" → false (unchanged)

"yes" → still invalid

Documentation
No breaking changes. Behavior is now more permissive and consistent with other coercive types.

[AnslemHack]

@Marsup here's a small PR to fix boolean validation around trailing input trims values and enforces 'true'/'false' only. Mind taking a look?