fix: correct package test import and mocks

[AlwinXue]

Hi mentors! 👋

This PR addresses the test and control-flow issues reported in Issue #15.

The Problem:
The package test file imported get_bids_metadata using a broken relative import, which prevented the tests from running against the real module. In addition, the tests mocked get_sequence() as if it could return None, but the actual factory raises ValueError for unsupported modalities or unmatched sequence classes. That made the tests assert an impossible execution path and left a dead if sequence is None branch in get_bids_metadata().

The Solution:
Correct Import: Updated the package test to import get_bids_metadata from pyaslreport.main.
Realistic Error Mocks: Replaced impossible return_value=None mocks with ValueError side effects that match the real behavior of get_sequence().
Dead Branch Removal: Removed the unreachable if sequence is None check from get_bids_metadata().
Stronger Assertions: Updated the tests to assert the actual error messages for unsupported modality and no matching sequence cases.

Validation:
Ran python -m pytest package/src/pyaslreport/tests/test_package.py -q and all 4 tests passed.

Fixes #15