fix: use pdesystem for AbstractPDESolution SII

[AayushSabharwal]

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

Add any other context about the problem here.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (855b485) 26.38% compared to head (72a8576) 5.27%.

Files	Patch %	Lines
src/solutions/solution_interface.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #565       +/-   ##
==========================================
- Coverage   26.38%   5.27%   -21.12%     
==========================================
  Files          53      53               
  Lines        4086    4059       -27     
==========================================
- Hits         1078     214      -864     
- Misses       3008    3845      +837     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.