fix slow & failing callback in test functions of ``test_responses.py``

[nishika26]

Describe the bug
While running tests, the following was observed

2025-07-20 23:39:11,434 - [N/A] - ERROR - app.api.routes.threads - Callback failed: 403 Client Error: Forbidden for url: http://example.com/callback
Traceback (most recent call last):
  File "/Users/nishikayadav/Desktop/platform/backend/app/api/routes/threads.py", line 44, in send_callback
    response.raise_for_status()
  File "/Users/nishikayadav/Desktop/platform/backend/.venv/lib/python3.12/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: http://example.com/callback
2025-07-20 23:39:11,446 - [N/A] - INFO - app.api.routes.responses - Callback sent successfully, assistant=assi********ific, project_id=195

✅ `test_responses_endpoint_success` took **75.67 seconds** to run.

Two main issues need to be addressed:

• The callback to http://example.com/callback is failing with a 403 Forbidden error, but is still logged as "sent successfully".
• The test function test_responses_endpoint_success is taking over one minute to run, which is unusually long for a unit test and slows down the test suite.

Suggested Solution:
Mock the external HTTP call to the callback endpoint (e.g., using pytest-mock, responses, or similar library) so that:

• The test does not make a real network request.

• The mocked response can simulate a successful (200 OK) callback.

• The test remains fast and reliable.