fix: Normalize openai client stop reason to make more robust #3400

Job	Run time
check-proto-changes-python	1m 6s
docs-example-check (./packages/autogen-core)	2m 34s
docs (./packages/autogen-core)	1m 25s
codecov (./packages/autogen-core)	1m 5s
codecov (./packages/autogen-magentic-one)	2m 0s
format	57s
lint	48s
codecov (./packages/autogen-ext)	2m 47s
markdown-code-lint	2m 18s
codecov (./packages/autogen-agentchat)	1m 19s
mypy (./packages/autogen-core)	2m 3s
pyright (./packages/autogen-core)	55s
mypy (./packages/autogen-magentic-one)	2m 38s
pyright (./packages/autogen-magentic-one)	1m 3s
mypy (./packages/agbench)	1m 2s
pyright (./packages/agbench)	1m 5s
samples-code-check	1m 3s
mypy (./packages/autogen-ext)	1m 56s
test (./packages/autogen-core)	54s
pyright (./packages/autogen-ext)	57s
mypy (./packages/autogen-agentchat)	1m 9s
test (./packages/autogen-magentic-one)	1m 34s
pyright (./packages/autogen-agentchat)	1m 21s
mypy (./packages/magentic-one-cli)	1m 16s
test (./packages/autogen-ext)	2m 47s
pyright (./packages/magentic-one-cli)	52s
test (./packages/autogen-agentchat)	56s
	39m 50s

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