Add non_boolean_string validator

Implement a new provider::validatefx::non_boolean_string validator.

Description:

• Validate that a string is not a boolean-like literal (true or false).

Acceptance criteria:

1. Subtask: Implement non_boolean_string validator in internal/validators
2. Subtask: Add unit tests for non_boolean_string validator
3. Subtask: Add non_boolean_string function to internal/functions package
4. Subtask: Add unit tests for non_boolean_string function
5. Subtask: Register non_boolean_string in registry.go
6. Subtask: Add documentation in examples/functions/non_boolean_string/function.tf
7. Subtask: Update docs table using make docs command
8. Subtask: Add integration test in integration/main.tf