Backport PR #1417 on branch 2.x (Add VertexAI model provider)

[meeseeksmachine]

Backport PR #1417: Add VertexAI model provider

[dlqqq]

Oh this needs the fix from #1412. Let me add that...

[dlqqq]

The absolute state of the pip package solver... 😮‍💨

I think a version floor is required for this job to pass.

[dlqqq]

These are the constraints being used in the "minimum dependencies" job at 31169dc:

2025-07-21T16:39:44.7373877Z click==8.1.0
2025-07-21T16:39:44.7374555Z importlib-metadata==5.2.0
2025-07-21T16:39:44.7390034Z jsonpath-ng==1.5.3
2025-07-21T16:39:44.7390496Z langchain-community==0.3.0
2025-07-21T16:39:44.7390957Z langchain==0.3.0
2025-07-21T16:39:44.7391347Z pydantic==2.10.0
2025-07-21T16:39:44.7391733Z cohere==5.12
2025-07-21T16:39:44.7392270Z pre-commit==3.3.3
2025-07-21T16:39:44.7392675Z deepmerge==2.0
2025-07-21T16:39:44.7393080Z faiss-cpu==1.8.0
2025-07-21T16:39:44.7393343Z jupyter-server==2.4.0
2025-07-21T16:39:44.7393576Z traitlets==5.6
2025-07-21T16:39:44.7393812Z syrupy==4.0.8

These constraints all look reasonably up-to-date, so I'm not sure what is causing the package solver to stall endlessly in CI. Notably, the boto3 resolution takes a very long time. It may be because cohere has a dependency on boto3 for some reason, so I will add version constraints on langchain_aws and langchain_cohere.

We may need to backport the upgrade to uv before merging this to have the package solver work faster if this does not work.

[dlqqq]

Sweet, the pins on langchain_aws and langchain_cohere fixed CI. Proceeding to merge.

There should be no user-facing changes introduced by the constraints I added. The new constraints were already implied by our existing langchain>=0.3,<0.4 constraint.