Refactor AssistantAgent on_message_stream (#5642) #5138

Job	Run time
lint	55s
mypy (./packages/autogen-core)	3m 31s
pyright (./packages/autogen-core)	1m 0s
test (./packages/autogen-core)	1m 15s
mypy (./packages/agbench)	59s
mypy (./packages/autogen-ext)	3m 18s
test (./packages/autogen-ext)	4m 16s
pyright (./packages/agbench)	58s
test (./packages/autogen-agentchat)	1m 22s
mypy (./packages/autogen-agentchat)	1m 37s
test-grpc	1m 25s
pyright (./packages/autogen-ext)	1m 13s
mypy (./packages/magentic-one-cli)	1m 27s
pyright (./packages/autogen-agentchat)	1m 1s
pyright (./packages/magentic-one-cli)	1m 1s
check-proto-changes-python	1m 16s
docs-example-check (./packages/autogen-core)	3m 50s
docs (./packages/autogen-core)	1m 48s
markdown-code-lint	3m 20s
samples-code-check	59s
format	1m 6s
codecov (./packages/autogen-core)	10s
codecov (./packages/autogen-ext)	10s
codecov (./packages/autogen-agentchat)	7s
codecov (autogen-ext-grpc)	8s
	38m 12s

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