PR: fix(sqlgetdata): read large NVARCHAR/VARBINARY correctly via incremental SQLGetData fix #12

Summary

Fix corrupted strings returned by the Dart ODBC binding when drivers return column data in chunks. Implement incremental reads for SQLGetData and correct byte/unit handling for wide chars and binaries.

Problem

Some ODBC drivers (notably SQL Server) return column data in multiple parts. The previous implementation:

• mixed bytes and UTF‑16 code unit counts when calling SQLGetData
• always requested wide-char and misinterpreted returned length
• did not loop to read remaining data
  This led to garbage bytes inside Dart strings.

Fix

• Loop-call SQLGetData until the driver signals completion (handle partial reads).
• Pass buffer sizes in bytes to SQLGetData.
• For binary columns use SQL_C_BINARY and read into a Uint8List.
• For wide-char (UTF-16) interpret returned length as bytes and convert by dividing by sizeof(Uint16).
• Properly handle SQL_NULL_DATA, avoid double-free and fix memory handling.

Files changed

• lib/src/dart_odbc_base.dart — incremental-read logic, byte/UTF‑16 handling, memory fixes.
• test/incremental_sqlgetdata_test.dart — integration test that reads a long NVARCHAR and asserts no long runs of non-printable characters.

Test (local)

1. Put credentials in example/.env
2. From repo root:

dart pub get
dart test test/incremental_sqlgetdata_test.dart -r expanded