convert : force setting sliding_window from original config (#15867)

convert : force setting sliding_window from original config (#15867)

* convert : force setting sliding_window from original config

This commit modifies the set_gguf_parameters method for EmbeddingGemma
so that it reads the sliding_window parameter from the original model
config.json and uses that value.

The motivation for this change is that the Gemma3TextConfig
constructor adjusts the sliding_window value, which can lead to
inconsistencies when converting models as we expects this value to
match the original model's configuration.

Refs: https://github.com/huggingface/transformers/blob/bb45d3631ec7026db04a77d33a52b31766372160/src/transformers/models/gemma3/configuration_gemma3.py#L230

* fix flake8 error

* add link to huggingface PR