Enhance error handling in `ImageRequest` class

[mandlinsarah]

Improved the error handling in the ImageRequest class by using the raise_for_status() method to automatically raise exceptions for HTTP errors. This change makes the code cleaner and improves reliability.

[jenuk]

Hi, thanks for taking time to improve the codebase.

With your implementation the specifics of the errors are lost. E.g. if you have wrong api key set, you will get

# previously
Traceback (most recent call last):
[...]
  File "[...]/black-forest-labs/flux/src/flux/api.py", line 99, in __init__
    self.request()
  File "[...]/black-forest-labs/flux/src/flux/api.py", line 118, in request
    raise ApiException(status_code=response.status_code, detail=result.get("detail"))
__main__.ApiException: ApiException(self.status_code=403, message='Not authenticated', detail=Not authenticated)

# your implementation
Traceback (most recent call last):
[...]
  File "[...]/black-forest-labs/flux/src/flux/api.py", line 99, in __init__
    self.request()
  File "[...]/black-forest-labs/flux/src/flux/api.py", line 116, in request
    response.raise_for_status()  # Automatically raises an HTTPError for bad responses
  File "[...]/black-forest-labs/flux/.venv/lib/python3.10/site-packages/requests/models.py", line 1024, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 403 Client Error: Forbidden for url: https://api.bfl.ml/v1/image

or when having an invalid request_id (as an example for any other kind of error):

# previously
Traceback (most recent call last):
  [...]
  File "[...]/black-forest-labs/flux/src/flux/api.py", line 169, in url
    result = self.retrieve()
  File "[...]/black-forest-labs/flux/src/flux/api.py", line 147, in retrieve
    raise ApiException(status_code=200, detail=f"API returned status '{result['status']}'")
__main__.ApiException: ApiException(self.status_code=200, message="API returned status 'Task not found'", detail=API returned status 'Task not found')

# your implementation
Traceback (most recent call last):
[...]
  File "[...]/black-forest-labs/flux/src/flux/api.py", line 167, in url
    result = self.retrieve()
  File "[...]/black-forest-labs/flux/src/flux/api.py", line 138, in retrieve
    response.raise_for_status()  # Automatically raises an HTTPError for bad responses
  File "[...]/black-forest-labs/flux/.venv/lib/python3.10/site-packages/requests/models.py", line 1024, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://api.bfl.ml/v1/get_result?id=210

Having this specific error here makes it clear that something went wrong with the API call and the reason is human-readable.

This change [...] improves reliability.

Did you run into any cases where the code raised any exceptions other than the ApiException? I'm not sure how the reliability would be increased by these changes.