Fix Recursion Issue in OCIDatascienceModel Due to is_model_by_reference Conflict in New OCI SDK

[mrDzurb]

Description

This PR resolves a recursion issue that occurred when creating a new model in Model Catalog (MC) with the new OCI SDK version.

Issue:

The new oci.data_science.models.Model introduced an is_model_by_reference field, which conflicts with the existing is_model_by_reference property in OCIDatascienceModel.
Since OCIDatascienceModel inherits from oci.data_science.models.Model, having the same attribute name caused recursive calls when serializing the model using sanitize_for_serialization from the OCI SDK.
This led to failures when attempting to create a new model in MC.

Fix:

Renamed is_model_by_reference to _is_model_by_reference in OCIDatascienceModel to prevent recursion and ensure compatibility with the new OCI SDK.

Impact:

• Fixes model creation failures in Model Catalog.
• Ensures compatibility with the new OCI SDK.
• Maintains backward compatibility with existing MC records.

Test

from ads.model.service.oci_datascience_model import OCIDataScienceModel

OCI_MODEL_PAYLOAD = {
    "compartment_id": "ocid1.compartment.oc1..<unique_ocid>",
    "project_id": "ocid1.datascienceproject.oc1.iad.<unique_ocid>",
    "display_name": "Generic Model With Small Artifact new",
    "description": "The model description",
    "lifecycle_state": "ACTIVE",
    "created_by": "ocid1.user.oc1..<unique_ocid>",
    "freeform_tags": {"key1": "value1"},
    "defined_tags": {"key1": {"skey1": "value1"}},
    "time_created": "2025-08-24T17:07:39.200000Z",
}

OCIDataScienceModel(**OCI_MODEL_PAYLOAD).to_dict()

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[qiuosier]

Actually in the OCI SDK, attributes are always defined with both attr and _attr. Both is_model_by_reference and _ is_model_by_reference are defined in the new SDK. The change in this PR will still cause a conflict. We need to rename this attribute to something else.