fix(python): Ensure assert_frame_not_equal and assert_series_not_equal raise on mismatched input types

[alexander-beedie]

Closes #18401.

Ensures both of the "not_equal" asserts also raise on mismatched input types, as their "equal" counterparts do; while such inputs are indeed technically not equal, that is not what the functions are testing.

Added quite a bit of bonus test coverage for the "not_equal" asserts while I was at it, along with additional validation of the expected assert error messages.

[stinodego]

Makes a lot of sense, thanks for the improvement 👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.88%. Comparing base (fb77bad) to head (2824444).
Report is 22 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18402   +/-   ##
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  Coverage   79.87%   79.88%           
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  Files        1495     1495           
  Lines      200210   200216    +6     
  Branches     2867     2867           
=======================================
+ Hits       159922   159941   +19     
+ Misses      39742    39729   -13     
  Partials      546      546           

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