[BUG]: /get_boolq_hard endpoint always crashes with ValueError due to invalid answer_style

### Bug Description

Bug Description :---

The `/get_boolq_hard` endpoint in [server.py] always raises a `ValueError` on every
request because it passes `answer_style="true_false"` which is not a valid value.

Root Cause :---

In [server.py] generated = qg.generate(
article=input_text,
num_questions=input_questions,
answer_style="true_false" #issue

In line 468 of Generator/main.py the only valid values accepted are ["all", "sentences", "multiple_choice"]

### Steps to Reproduce

Send any POST request to /get_boolq_hard with valid input.

### Logs and Screenshots

_No response_

### Environment Details

OS:Windows 11
Python:3.11.9

### Impact

Medium - Feature works but has issues

### Code of Conduct

- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates