[Keras 3 OpenVINO Backend]: Improve support for `unstack` operations

[daehyun99]

Request Description

This issue tracks improving support for unstack in the Keras OpenVINO backend.

The operation is currently not implemented properly in keras/backend/openvino

I will submit a PR implementing this and enable the corresponding tests.

Ref: #34017

Feature Use Case

No response

Issue submission checklist

• The feature request or improvement must be related to OpenVINO

[daehyun99]

.take