Fix vertex_ai_index_endpoint_deployed_index import without provider region

[Shuanglu]

At terraform import time the .tf config is not yet applied to the schema, so d.Get("region") is empty and tpgresource.GetRegion falls through to the provider config. When provider-level region/zone is unset, Read fails with "Cannot determine region" because the URL is templated on {{region}}. Parse region (and project) from the index_endpoint path in the import ID and d.Set them so Read can resolve the URL.

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vertexai: fixed `terraform import` of `google_vertex_ai_index_endpoint_deployed_index` failing with "Cannot determine region" when provider-level `region`/`zone` is unset

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