[KNITRO] Remove iteration count for MIP

[eminyouskn]

Fixes # .

Summary/Motivation:

This PR updates the iteration count handling for KNITRO. Since the iteration count is not meaningful for MIP problems, the solver now returns None in those cases.

Changes proposed in this PR:

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.41%. Comparing base (168e961) to head (8174acb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3802      +/-   ##
==========================================
- Coverage   89.41%   89.41%   -0.01%     
==========================================
  Files         909      909              
  Lines      105579   105586       +7     
==========================================
+ Hits        94408    94414       +6     
- Misses      11171    11172       +1     

Flag	Coverage Δ
builders	29.10% <12.50%> (-0.01%)	⬇️
default	86.04% <100.00%> (?)
expensive	35.76% <12.50%> (?)
linux	86.72% <12.50%> (-2.46%)	⬇️
linux_other	86.72% <12.50%> (-0.01%)	⬇️
osx	82.88% <12.50%> (-0.01%)	⬇️
win	84.98% <12.50%> (-0.01%)	⬇️
win_other	84.98% <12.50%> (-0.01%)	⬇️

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