feat: enhance Gemini model support in OpenAI client and tests #4563

Job	Run time
lint	1m 4s
format	1m 4s
docs (./packages/autogen-core)	1m 39s
docs-example-check (./packages/autogen-core)	3m 26s
mypy (./packages/autogen-core)	3m 32s
pyright (./packages/autogen-core)	1m 6s
mypy (./packages/agbench)	1m 8s
pyright (./packages/agbench)	53s
mypy (./packages/autogen-ext)	3m 4s
pyright (./packages/autogen-ext)	1m 4s
mypy (./packages/autogen-agentchat)	1m 34s
pyright (./packages/autogen-agentchat)	54s
mypy (./packages/magentic-one-cli)	1m 27s
pyright (./packages/magentic-one-cli)	1m 2s
samples-code-check	1m 19s
test (./packages/autogen-core)	1m 4s
test (./packages/autogen-ext)	3m 38s
test (./packages/autogen-agentchat)	1m 16s
test-grpc	1m 50s
check-proto-changes-python	1m 24s
markdown-code-lint	2m 54s
codecov (./packages/autogen-core)	8s
codecov (./packages/autogen-ext)	8s
codecov (./packages/autogen-agentchat)	8s
codecov (autogen-ext-grpc)	8s
	36m 54s

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