OPTIMIZATION #1: Direct PyUnicode_DecodeUTF16 for NVARCHAR conversion (Linux/macOS)

Problem:
- Linux/macOS performed double conversion for NVARCHAR columns
- SQLWCHAR → std::wstring (via SQLWCHARToWString) → Python unicode
- Created unnecessary intermediate std::wstring allocation

Solution:
- Use PyUnicode_DecodeUTF16() to convert UTF-16 directly to Python unicode
- Single-step conversion eliminates intermediate allocation
- Platform-specific optimization (Linux/macOS only)

Impact:
- Reduces memory allocations for wide-character string columns
- Eliminates one full conversion step per NVARCHAR cell
- Regular VARCHAR/CHAR columns unchanged (already optimal)

Author: bewithgaurav

mssql_python/pybind/ddbc_bindings.cpp

@@ -3294,8 +3294,19 @@ SQLRETURN FetchBatchData(SQLHSTMT hStmt, ColumnBuffers& buffers, py::list& colum
                     if (!isLob && numCharsInData < fetchBufferSize) {
 #if defined(__APPLE__) || defined(__linux__)
                         SQLWCHAR* wcharData = &buffers.wcharBuffers[col - 1][i * fetchBufferSize];
-                        std::wstring wstr = SQLWCHARToWString(wcharData, numCharsInData);
-                        row[col - 1] = wstr;
+                        // OPTIMIZATION #1: Direct UTF-16 decode - eliminates intermediate std::wstring
+                        PyObject* pyStr = PyUnicode_DecodeUTF16(
+                            reinterpret_cast<const char*>(wcharData),
+                            numCharsInData * sizeof(SQLWCHAR),
+                            NULL,  // errors - use default handling
+                            NULL   // byteorder - auto-detect
+                        );
+                        if (pyStr) {
+                            row[col - 1] = py::reinterpret_steal<py::object>(pyStr);
+                        } else {
+                            PyErr_Clear();
+                            row[col - 1] = std::wstring(L"");
+                        }
 #else
                         row[col - 1] = std::wstring(
                             reinterpret_cast<wchar_t*>(&buffers.wcharBuffers[col - 1][i * fetchBufferSize]),