Fix SessionDataSet: row count, async dispose, and null measurement issues (#53, #54, #55) (#56)

* Fix SessionDataSet: row count, async dispose, and null measurement issues (#53, #54, #55)
修复 SessionDataSet 行数统计、异步释放及空值测点问题

- Fix CurrentBatchRowCount() always returning 0 by eagerly constructing
  the first TsBlock in RpcDataSet constructor when initial data is available
  修复 CurrentBatchRowCount() 始终返回 0 的问题，在构造函数中预先反序列化首个 TsBlock

- Add IAsyncDisposable to SessionDataSet and RpcDataSet, providing
  DisposeAsync() that properly awaits Close() to avoid sync-over-async deadlocks
  为 SessionDataSet 和 RpcDataSet 添加 IAsyncDisposable 接口，支持 await using 语法

- Fix GetRow() including null-valued columns in RowRecord by using
  IsNull() check before calling type-specific getters, instead of relying
  on value type null checks which always pass for int/bool/float/etc.
  修复 GetRow() 中值类型默认值绕过 null 检查导致空值列被错误包含的问题

* Fix SessionDataSet.Close() idempotency and add regression tests
修复 SessionDataSet.Close() 幂等性问题并添加回归测试

- Set _isClosed = true in Close() finally block to prevent
  NullReferenceException on repeated Close/Dispose/DisposeAsync calls
  在 Close() 的 finally 块中设置 _isClosed = true，防止重复调用导致空引用异常

- Add RpcDataSetTests covering:
  - CurrentBatchRowCount returns correct size before first row read
  - CurrentBatchRowCount returns 0 when no data
  - GetRow excludes null-valued measurements for value types (BOOLEAN, INT32, DOUBLE)
  - DataTypes/Measurements/Values lists stay consistent
  添加 RpcDataSet 回归测试覆盖行数统计和空值测点排除