Fix QA max_answer_len validation error message

[antznette1]

Summary
This PR fixes a small typo in QuestionAnsweringPipeline._sanitize_parameters() where the ValueError raised for invalid max_answer_len was missing a closing parenthesis in the "(got ...)" portion of the message.

Changes

• Fix malformed ValueError message for max_answer_len validation in:
  • src/transformers/pipelines/question_answering.py
• Add a lightweight regression test (no model downloads) that asserts the error message is well-formed:
  • tests/pipelines/test_pipelines_question_answering.py

Testing

• python -m pytest tests/pipelines/test_pipelines_question_answering.py -k sanitize_parameters_max_answer_len_error_message -q