refactor code from using vertexai.preview to vertexai

[hilliao]

code files like this should migrate from from vertexai.preview import rag to from vertexai import rag. Observed:

def upload_pdf_to_corpus(corpus_name, pdf_path, display_name, description):
  """Uploads a PDF file to the specified corpus."""
  print(f"Uploading {display_name} to corpus...")
  try:
    rag_file = rag.upload_file(
        corpus_name=corpus_name,
        path=pdf_path,
        display_name=display_name,
        description=description,
    )

expect:

response = rag.import_files(
    corpus_name=corpus_name,
    paths=paths,
    transformation_config=rag.TransformationConfig(
        rag.ChunkingConfig(chunk_size=1024, chunk_overlap=256)
    ),
    import_result_sink="gs://sample-existing-folder/sample_import_result_unique.ndjson",  # Optional: This must be an existing storage bucket folder, and the filename must be unique (non-existent).
    llm_parser=rag.LlmParserConfig(
      model_name="gemini-2.5-pro-preview-05-06",
      max_parsing_requests_per_min=100,
    ),  # Optional
    max_embedding_requests_per_min=900,  # Optional
)
print(f"Imported {response.imported_rag_files} files.")

[cnaples79]

I'm working on this.