Fix: Integrate offline validation for SBase unit assignments

[dyrpsf]

Resolve TODO: Integrate offline validation for SBase unit assignments

Description
This PR resolves the pending TODO in AbstractNamedSBaseWithUnit.java by shifting unit assignment validation to utilize the JSBML offline validation framework.

Previously, programmatic assignment of an invalid unit triggered a hard IllegalArgumentException, which disrupted model building. This update ensures that invalid unit assignments now gracefully register an SBMLError within the associated SBMLDocument's error log (tracking SBMLErrorCodes.CORE_10311 syntax requirements), maintaining execution flow while strictly enforcing SBML specifications.

Architectural Changes

• Validation Shift: Updated setUnits(String) to check for a valid SBMLDocument context and append an SBMLError upon invalid unit assignment.
• Isolated Node Handling: Maintained the IllegalArgumentException fallback for isolated nodes lacking a document context.
• Test Verification: Verified successfully against TestAbstractNamedSBaseWithUnits using Maven targeted testing.

cc: @draeger