Fix code quality issues in SqlDecimal migration

[Copilot]

Addresses code quality issues found during review of the decimal → SqlDecimal migration for full 38-digit SQL Server NUMERIC/DECIMAL precision support.

Changes

• Fix broken indentation in ExtractNumericColumn — The if/else pattern had the else block body at the wrong nesting level. Restored early-return guard clause, which is both correct and consistent with the rest of the codebase.

• Replace magic number with named constant — *strLenOrNullMap = 19 → *strLenOrNullMap = SqlNumericStructSize in CSharpParamContainer.cs. The constant already exists in Sql.cs and is used elsewhere for the same purpose.

• Remove unused import and stray blank line in SqlNumericHelper.cs (System.Linq was never referenced).

Before/After (ExtractNumericColumn)

// Before: broken indentation, else block body not indented
if (column == null)
{
    SetDataPtrs<SqlNumericStruct>(columnNumber, Array.Empty<SqlNumericStruct>());
}
else
{
            // ← code at wrong indent level
byte precision = 0;
...
                // ← inconsistent
                SetDataPtrs<SqlNumericStruct>(columnNumber, numericArray);
            }

// After: early return, flat structure
if (column == null)
{
    SetDataPtrs<SqlNumericStruct>(columnNumber, Array.Empty<SqlNumericStruct>());
    return;
}

byte precision = 0;
...
SetDataPtrs<SqlNumericStruct>(columnNumber, numericArray);

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