Add solver input validation and test scripts

[ferrari212]

Description

This PR introduces an initial regression and unity testing structure for FEAScript-core as part of the ongoing improvements proposed in #50.

The main goal is to establish a foundation for validating existing functionality and preventing unintended changes in future development. These tests provide baseline coverage that can be expanded as new features, bug fixes, and improvements are introduced.

Changes

• Added an initial regression test structure.
• Added baseline test cases to verify existing behavior.
• Added reference cases for future comparison and validation.
• Improved the testing workflow to support future development and refactoring.

Motivation

As FEAScript-core continues to evolve, maintaining a reliable regression testing suite is important to ensure that existing functionality remains stable while new capabilities are added.

This PR represents a first step toward broader automated validation coverage and provides a framework where additional regression cases can be included incrementally.

Testing

• Add regression tests covering initial scenarios.
• Add unity tests covering initial scenarios.
• Verified that the current implementation passes the added tests.

Related Issue

Partially addresses #50