add openai in reqs

[sfc-gh-rroy]

Description

Please include a summary of the changes and the related issue that can be
included in the release announcement. Please also include relevant motivation
and context.

Other details good to know for developers

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Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• New Tests
• This change includes re-generated golden test results
• This change requires a documentation update

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Important

Add langchain-openai to optional test dependencies and update version constraints in pyproject.toml files for compatibility with different Python versions.

• Dependencies:
  • Add langchain-openai to env-tests-optional target in Makefile.
• Version Constraints:
  • Update langchain, langchain-core, and pydantic version constraints in src/apps/langchain/pyproject.toml and src/providers/langchain/pyproject.toml for Python <3.9 and >=3.9.
  • Update langchain-community and pydantic version constraints in src/providers/openai/pyproject.toml for Python <3.9 and >=3.9.

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