Tests fixes due to optional openai dependancy

[davorrunje]

Why are these changes needed?

Related issue number

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation 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.

[codecov]

Codecov Report

Attention: Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
autogen/import_utils.py	92.30%	1 Missing and 1 partial ⚠️
...t/realtime/experimental/clients/oai/base_client.py	85.71%	0 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
...hat/realtime/experimental/clients/gemini/client.py	48.59% <100.00%> (ø)
...t/realtime/experimental/clients/oai/base_client.py	83.90% <85.71%> (+30.96%)	⬆️
autogen/import_utils.py	89.75% <92.30%> (-0.09%)	⬇️

... and 27 files with indirect coverage changes