Fix the endless loop in Join and Merge when both suffixes are the same

[HuzaifaChaudary]

Fixes #6128

Join and Merge rename a column that appears in both frames . the existing one gets
leftSuffix , the new one gets rightSuffix , and it repeats until the two names are
different . when both suffixes are the same string the names stay equal after every round , so
nothing ever ends the loop and both names keep growing .

the reporter hit it by passing the join column name where the suffix goes . michaelgsharp
already named the cause on the issue in 2022 , this is just the patch for it .

it now throws an ArgumentException naming the two parameters and the column .

the check is inside SetSuffixForDuplicatedColumnNames rather than up front in Join and
Merge , because equal suffixes are fine when the two frames share no column name . nothing
gets renamed in that case and it works today , so an up front check would break it . i
measured that before deciding where to put it :

join , shared column name , same suffix        hangs , all four algorithms
join , no shared column name , same suffix     fine
merge , shared column name , same suffix       hangs
merge , default suffixes                       fine

three tests added . the first covers all four JoinAlgorithm values with "_same"/"_same"
and with ""/"" , then checks "_left"/"_right" still gives the full column set . the second
goes the other way and pins that equal suffixes are still allowed when nothing collides . the
third is the same thing for Merge .

worth saying how the without-the-fix run looks , since it is not an ordinary failure . the
defect is an endless loop , so the tests do not go red , they never finish . i took the fix out
and the run was still going after 150 seconds with no result , at about 92 percent cpu . with
the fix the same six tests finish in 43 ms .

Microsoft.Data.Analysis.Tests is 483 passing , 0 failures , 5 skipped . the tree without this
runs 477 , so the only difference is the six new ones . build is clean with
EnforceCodeStyleInBuild.

i also grepped every .Join( and .Merge( caller in src and test , none of them pass
equal suffixes , so the new guard cannot fire on existing code .

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