fix: Normalize openai client stop reason to make more robust #3389

Job	Run time
docs (./packages/autogen-core)	1m 36s
check-proto-changes-python	1m 19s
docs-example-check (./packages/autogen-core)	2m 33s
codecov (./packages/autogen-core)	59s
format	59s
codecov (./packages/autogen-magentic-one)	1m 58s
codecov (./packages/autogen-ext)	2m 45s
lint	52s
markdown-code-lint	2m 15s
codecov (./packages/autogen-agentchat)	1m 14s
mypy (./packages/autogen-core)	2m 14s
pyright (./packages/autogen-core)	1m 1s
mypy (./packages/autogen-magentic-one)	1m 14s
samples-code-check	58s
pyright (./packages/autogen-magentic-one)	1m 5s
test (./packages/autogen-core)	2m 4s
mypy (./packages/agbench)	1m 9s
pyright (./packages/agbench)	55s
mypy (./packages/autogen-ext)	1m 55s
test (./packages/autogen-magentic-one)	1m 22s
mypy (./packages/autogen-agentchat)	1m 27s
pyright (./packages/autogen-ext)	59s
test (./packages/autogen-ext)	2m 53s
pyright (./packages/autogen-agentchat)	55s
mypy (./packages/magentic-one-cli)	1m 9s
test (./packages/autogen-agentchat)	1m 3s
pyright (./packages/magentic-one-cli)	1m 4s
	39m 57s

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