Document offline validation as the default and add test (#246)

[dyrpsf]

This PR addresses #246.

Summary

SBMLDocument.checkConsistency() already delegates to checkConsistencyOffline(), so offline validation is effectively the default. However, the Javadoc still claimed that validation was "currently performed using the SBML.org online validator". This PR:

• updates the documentation to match the actual behaviour, and
• adds a small regression test to ensure checkConsistency() continues to use the offline validator.

Changes

1. Javadoc updates in SBMLDocument

   • core/src/org/sbml/jsbml/SBMLDocument.java
     • checkConsistencyOffline() Javadoc now explains that the offline validator is used by default when calling checkConsistency(), and points to checkConsistencyOnline() as the alternative.
     • checkConsistency() Javadoc is updated to state that validation is performed using the JSBML internal offline validator, with the SBML.org online validator still available via checkConsistencyOnline().

   Note: No executable code was changed; only comments/Javadoc were modified.
   The method remains:

   public int checkConsistency() {
     return checkConsistencyOffline();
   }

2. New unit test

   • Added core/test/org/sbml/jsbml/test/SBMLDocumentValidationModeTest.java
   • The test creates a small TestDocument subclass of SBMLDocument that overrides:
     • checkConsistencyOffline() and
     • checkConsistencyOnline()
       and records which one is called.
   • The test checkConsistencyUsesOfflineValidatorByDefault() asserts that:
     • checkConsistency() calls the offline variant,
     • does not call the online variant,
     • and returns the offline method’s result.

   This guards against future regressions where checkConsistency() might accidentally call the online validator again.

Tests

Executed locally:

• mvn -pl core test ✅

Test Results

The newly added regression test behaves as expected and passes successfully.