Add mcp_proxy for bidirectional message forwarding #3883

Job	Run time
checks / readme-snippets	15s
checks / pre-commit	47s
checks / test (3.10, lowest-direct, windows-latest)	2m 22s
checks / test (3.11, lowest-direct, windows-latest)	2m 28s
checks / test (3.10, lowest-direct, ubuntu-latest)	10m 15s
checks / test (3.12, lowest-direct, ubuntu-latest)	47s
checks / test (3.10, highest, ubuntu-latest)	51s
checks / test (3.12, highest, ubuntu-latest)	52s
checks / test (3.11, lowest-direct, ubuntu-latest)	42s
checks / test (3.13, highest, windows-latest)	3m 4s
checks / test (3.10, highest, windows-latest)	2m 36s
checks / test (3.12, lowest-direct, windows-latest)	3m 13s
checks / test (3.11, highest, ubuntu-latest)	47s
checks / test (3.12, highest, windows-latest)	3m 39s
checks / test (3.11, highest, windows-latest)	3m 43s
checks / test (3.13, lowest-direct, ubuntu-latest)	45s
checks / test (3.13, lowest-direct, windows-latest)	2m 31s
checks / test (3.13, highest, ubuntu-latest)	44s
	40m 21s

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