unit test suite and CI

[vtorlima]

Summary

Adds the first CI pipeline and a comprehensive unit test suite to the pyaslreport package. Before this, the package had no CI and only a single pre-existing test file covering get_bids_metadata orchestration. Without automated coverage, regressions in validation, parameter extraction, or report generation could go undetected.

What's included

Unit tests covering the internal processing logic:

• Field renaming and unit conversion.
• All six validator classes (Number, String, Boolean, NumberArray, NumberOrNumberArray, Consistency) at their schema-configured boundaries, plus a schema-loading smoke test
• M0 contradiction paths, TSV validation, and background-suppression warning branches
• File grouping for both BIDS (exact-suffix) and DICOM (substring) modes, including TSV header enforcement

CI via GitHub Actions:

• Runs the unit suite on Python 3.11 and 3.12
• Black and isort formatting gates (pinned in the test extra)