feat(pool_token_logic): transfer out remaining balance of underlying token when drop_reserve

[matchv]

• Add transfer logic of underlying token balance when drop_reserve()
• Implement proper treasury transfer mechanism for remaining underlying assets before reserve removal
• Add comprehensive test coverage to verify drop_reserve behavior across different time periods
• Validate treasury balance transfer and resource account cleanup in drop_reserve operations
• Improve test assertions to verify asset accounting accuracy

Test scenarios covered:

• 6 months, 1 year, 2 years, and 5 years interest accrual periods
• Verification of accrued_to_treasury and underlying balance
• Treasury balance transfer validation after drop_reserve execution

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.89%. Comparing base (6356330) to head (1604c74).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
- Coverage   97.08%   96.89%   -0.19%     
==========================================
  Files          16       16              
  Lines         514      515       +1     
==========================================
  Hits          499      499              
- Misses         15       16       +1     

Flag	Coverage Δ
move	96.89% <ø> (-0.19%)	⬇️

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