Fix: Properly await `agent.run()` in README `Hello World` example #3357

Job	Run time
codecov (./packages/autogen-core)	56s
format	52s
codecov (./packages/autogen-magentic-one)	1m 49s
codecov (./packages/autogen-ext)	2m 37s
codecov (./packages/autogen-agentchat)	1m 13s
lint	45s
mypy (./packages/autogen-core)	2m 22s
pyright (./packages/autogen-core)	1m 4s
mypy (./packages/autogen-magentic-one)	1m 8s
pyright (./packages/autogen-magentic-one)	1m 5s
mypy (./packages/agbench)	1m 4s
pyright (./packages/agbench)	1m 1s
mypy (./packages/autogen-ext)	1m 49s
pyright (./packages/autogen-ext)	57s
mypy (./packages/autogen-agentchat)	1m 7s
pyright (./packages/autogen-agentchat)	1m 3s
mypy (./packages/magentic-one-cli)	1m 7s
pyright (./packages/magentic-one-cli)	1m 12s
test (./packages/autogen-core)	52s
test (./packages/autogen-magentic-one)	1m 20s
check-proto-changes-python	1m 22s
test (./packages/autogen-ext)	2m 47s
docs (./packages/autogen-core)	1m 24s
test (./packages/autogen-agentchat)	54s
docs-example-check (./packages/autogen-core)	2m 56s
markdown-code-lint	1m 53s
samples-code-check	50s
	37m 29s

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