Add unit tests for HTTP provider exceptions

[gps23]

What does this PR do?

Adds unit tests for the HTTP provider exception classes:

• HttpErrorException
• HttpMethodException

The new tests verify that both exceptions inherit from AirflowException and preserve the expected error message.

Since the required tests now exist, this PR also removes
providers/http/tests/unit/http/test_exceptions.py
from the OVERLOOKED_TESTS list in the project structure test.

Testing

uv run pytest providers/http/tests/unit/http/test_exceptions.py -v
uv run pytest airflow-core/tests/unit/always/test_project_structure.py -k test_providers_modules_should_have_tests -v


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[X] Yes (ChatGPT)
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[boring-cyborg]

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