feat: [dotnet] simplify building/codecov CI #4376

Job	Run time
docs (./packages/autogen-core)	1m 40s
format	1m 3s
lint	1m 4s
mypy (./packages/autogen-core)	3m 38s
pyright (./packages/autogen-core)	1m 10s
mypy (./packages/agbench)	1m 12s
pyright (./packages/agbench)	1m 6s
mypy (./packages/autogen-ext)	3m 12s
samples-code-check	1m 15s
test (./packages/autogen-core)	1m 9s
pyright (./packages/autogen-ext)	2m 57s
mypy (./packages/autogen-agentchat)	1m 21s
pyright (./packages/autogen-agentchat)	1m 10s
test (./packages/autogen-ext)	4m 18s
mypy (./packages/magentic-one-cli)	1m 28s
pyright (./packages/magentic-one-cli)	57s
check-proto-changes-python	1m 31s
test (./packages/autogen-agentchat)	1m 19s
docs-example-check (./packages/autogen-core)	3m 49s
markdown-code-lint	2m 29s
codecov (./packages/autogen-core)	9s
codecov (./packages/autogen-ext)	12s
codecov (./packages/autogen-agentchat)	13s
	38m 22s

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