fix(openai): add async context manager support

[Vishnu Vardhan (Vishnu3568)]

Why are these changes needed?

Related issue number

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• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[Copilot]

Pull request overview

Adds async context manager support to the OpenAI model client so it can be used with async with ... to ensure underlying OpenAI resources are closed automatically, and adds tests to validate cleanup and exception propagation behavior.

Changes:

• Implement __aenter__/__aexit__ on BaseOpenAIChatCompletionClient to close the underlying async OpenAI client on context exit.
• Add pytest-asyncio tests that verify OpenAIChatCompletionClient can be used as an async context manager and that close() is awaited even when exceptions occur inside the block.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/packages/autogen-ext/tests/models/test_openai_model_client.py	Adds async context manager tests (normal flow + exception flow) for OpenAIChatCompletionClient.
python/packages/autogen-ext/src/autogen_ext/models/openai/_openai_client.py	Implements async context manager protocol (__aenter__/__aexit__) that calls close() on exit.

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