fix: copy dicom inputs before running dcm2niix

[AlwinXue]

Hi mentors! 👋

This PR addresses the DICOM conversion flow bug reported in Issue #40.

The Problem:
DICOM2NiFTIConverter.convert() created a temporary directory and then immediately checked whether it was empty, but it never copied the incoming DICOM files into that directory. As a result, the converter returned No DICOM files found. before dcm2niix could ever run.

The Solution:
Copy Inputs Into Temp Directory: Each uploaded DICOM file is now copied into the converter's temporary working directory before metadata extraction and subprocess execution.
Preserve Existing Metadata Logic: The existing SeriesNumber and lRepetitions extraction logic is kept intact.
Regression Coverage: Added a focused converter test that verifies the temporary input directory is populated, subprocess.run() is reached, and the generated JSON gets the expected lRepetitions value.

Validation:
Ran python -m pytest package/src/pyaslreport/tests/test_dicom_to_nifti_converter.py -q and the new converter regression test passed.

Fixes #40