[ShryukGrandhi]

Fixes #7381

PrimitiveColumnContainer<T>.CloneAs*Container mapped every source buffer to exactly one buffer of the target type. Buffer capacity is ArrayUtility.ArrayMaxSize / sizeof(T), so this breaks in two ways:

• When the target type is larger (e.g. int -> double), a full source buffer holds more values than a target buffer can. Cloning an Int32DataFrameColumn with more than ~268M values as double threw ArgumentException: ... exceeds buffer capacity. This is reachable from public APIs like Int32DataFrameColumn.Add(double).
• When the target type is smaller (e.g. long -> int), the result contains buffers that aren't full. Row lookups assume every buffer except the last holds exactly MaxCapacity values (rowIndex / MaxCapacity), so indexing into that result returns the wrong values.

Fix

The per-type copy loops are replaced with one private CloneAs<U>(Func<T, U> convert) helper. It fills target buffers up to ReadOnlyDataFrameBuffer<U>.MaxCapacity and starts a new buffer and null bitmap when one is full. Validity bits are copied into the matching new bitmap. When the column has no nulls they are set in bulk. Each CloneAs*Container method now just passes its converter to this helper. For bool, values from a non-bool column still become false and the validity bits are kept, as before.

Testing

• Added PrimitiveDataFrameColumn_Int32_CloneAsDouble_AcrossMultipleBuffers. It builds an Int32DataFrameColumn that spans two buffers, calls Add(0.5) (which clones as double) and checks values and nulls around the double and int buffer boundaries. It is marked X64Fact like the other large-allocation tests next to it. For local runs on arm64 I temporarily switched it to [Fact]: it failed before the change with the exceeds buffer capacity exception and passes after it.
• dotnet test test/Microsoft.Data.Analysis.Tests passes: 1765 passed, 6 skipped, 0 failed.
• dotnet format --verify-no-changes reports no changes for the touched files.

[codecov]

Codecov Report

❌ Patch coverage is 97.43590% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.77%. Comparing base (29f19e5) to head (ddebbdc).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
...icrosoft.Data.Analysis/PrimitiveColumnContainer.cs	96.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7819      +/-   ##
==========================================
- Coverage   70.77%   70.77%   -0.01%     
==========================================
  Files        1420     1420              
  Lines      279877   279775     -102     
  Branches    27604    27587      -17     
==========================================
- Hits       198077   198000      -77     
+ Misses      73620    73600      -20     
+ Partials     8180     8175       -5     

Flag	Coverage Δ
Debug	70.77% <97.43%> (-0.01%)	⬇️
production	64.20% <96.42%> (-0.01%)	⬇️
test	90.76% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...ta.Analysis.Tests/PrimitiveDataFrameColumnTests.cs	99.76% <100.00%> (+0.01%)	⬆️
...icrosoft.Data.Analysis/PrimitiveColumnContainer.cs	86.66% <96.42%> (+0.07%)	⬆️

... and 11 files with indirect coverage changes

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