Fix failure_cases index value for MultiIndex schema errors

[amerberg]

This fixes a regression in #2103 where the index column of failure_cases for schema errors raised in validating a multiindex level only includes the level that failed validation, rather than the tuple of all multi-index values. The fix implemented here is to validate the level as a series indexed by the full multi-index. A unit test is also added to prevent similar regressions in the future.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.67%. Comparing base (ae2ae88) to head (7c18d93).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2186       +/-   ##
===========================================
+ Coverage   81.51%   93.67%   +12.16%     
===========================================
  Files         137      137               
  Lines       10895    10898        +3     
===========================================
+ Hits         8881    10209     +1328     
+ Misses       2014      689     -1325     

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