chore(deps): bump openai support to v2+

[ashley-evans]

Updates package to bump minimum supported openai version to v2+.

In doing so, this has required dropping support for Python 3.9 as langchain-openai v1.0+ is required to support openai v2+.

Python 3.9 is EOL as of end of October.

Opted to raise the PR to set minimum to v2+ as:

• v2.0.0 has been released since Sep 30 2025
• Bumping minimum reduces maintenance overhead moving forward

If you'd prefer I can update to support >=1.66,<3.0 - I don't think the breaking change added in v2.0.0 impacts any of the code in this package.

It's worth noting that there are now a number of warnings raised when running tests, e.g.

examples/test_vector_stores.py::test_create_vector_store_from_thread_create
  openai-responses-python/examples/test_vector_stores.py:78: DeprecationWarning: The Assistants API is deprecated in favor of the Responses API
    thread = client.beta.threads.create(

I think it might be sensible to have a separate task to handle this deprecation before the API is removed on August 26, 2026.

[mharrisb1]

Thanks @ashley-evans I'll try to review, merge, and release today 👍