[preetha-intel]

Add GGML decoder to support OV GGUF Frontend.

Overview

This PR removes the legacy in-tree OpenVINO translation layer from ggml-openvino and moves the backend onto the OpenVINO GGUF frontend (openvino::frontend::gguf).

Until now the backend carried its own copy of the GGML→OpenVINO conversion: a decoder interface, node context, per-op translators, graph-fusion passes and a translate session, all under ggml/src/ggml-openvino/openvino/.
After this change, the frontend owns the generic op translations and passes. The backend keeps a GgmlOvDecoder bridge that exposes the ggml graph to the frontend, plus the runtime pieces (device handling, KV cache, compile and cache logic).

Changes

• Removed the legacy bridge. The whole ggml-openvino/openvino/ tree is deleted: decoder.h, frontend., input_model., node_context.h, translate_session., utils., all of op/* (about 55 translators) and the fusion passes in pass/*. These include fuse_to_sdpa, fuse_to_conv, fuse_moe_router, squeeze_matmul and the dequantization marking passes.
• Reworked GgmlOvDecoder (ggml-decoder.cpp/.h). It is now the bridge that exposes the ggml cgraph, tensor metadata and model-level info to the GGUF frontend.
• Simplified ggml-openvino.cpp and utils.cpp. Graph conversion now goes through the frontend, and the per-op support logic that duplicated the translators is gone.
• Moved the KV-state sequence-axis helper. It went from openvino/pass/kv_state_seq_axis.{h,cpp} to kv-state-seq-axis.{h,cpp} in the backend directory, because it is runtime logic and not a frontend pass.

CMake changes

• ggml-openvino now links openvino::frontend::gguf.
• Configuration fails with a clear error if that target isn't found.
• The source globs are no longer recursive, since the subdirectories are gone.

OpenVINO PR enabling this flow : openvinotoolkit/openvino#38769

This PR is dependent on OV PR to be merged.