Fix #67: Add offset methods for boundaries and include dispatch tests

[utkuyilmaz1903]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Resolves #67.

Implemented "Option A" as suggested by @xtalax to provide fallback offset implementations for boundary types.

Key details:

• Added offset methods directly to LowerBoundary and UpperBoundary (instead of AbstractLowerBoundary / AbstractUpperBoundary as originally suggested, since those abstract types are not defined in the current hierarchy and caused an UndefVarError during local testing).
• Added a new @safetestset to verify correct dispatch and offset calculations, preventing future regressions.