* feat: add Vertex AI support for file content retrieval
- Extended `custom_llm_provider` to include "vertex_ai" in `afile_content` function.
- Implemented file content retrieval logic for Vertex AI in `VertexAIFilesHandler`.
- Added helper method to extract bucket and object from URL-encoded file_id.
- Created comprehensive unit and integration tests for Vertex AI file handling.
- Updated transformation logic to ensure compatibility with Vertex AI file responses.
* fix: update Vertex AI file transformation logic
- Modified the transformation logic in `VertexAIFilesConfig` to return a newline-separated JSON string for batch JSONL files instead of a array if JSON strings.
* fix: enhance Vertex AI output handling in transformation logic
- Updated the transformation logic in `VertexAIBatchTransformation` to utilize the new `OutputInfo` TypedDict for retrieving the GCS output directory.
- Added `OutputInfo` class to type definitions for better structure and clarity in Vertex AI responses.
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