chore: update the vertexai inference impl to use openai-python for openai-compat functions

[mattf]

What does this PR do?

update VertexAI inference provider to use openai-python for openai-compat functions

Test Plan

$ VERTEX_AI_PROJECT=... uv run llama stack build --image-type venv --providers inference=remote::vertexai --run
...
$ LLAMA_STACK_CONFIG=http://localhost:8321 uv run --group test pytest -v -ra --text-model vertexai/vertex_ai/gemini-2.5-flash tests/integration/inference/test_openai_completion.py
...

i don't have an account to test this. get_api_key may also need to be updated per https://cloud.google.com/vertex-ai/generative-ai/docs/start/openai

[mattf]

@leseb here you go