client: deprecate inferring stdio transports from str, prefer Path (#4941)

* spike: refuse stdio inference from str

* client: infer stdio transports from Path only, never from str

🤖 Generated with Claude Code

* client: deprecate inferring stdio transports from str, prefer Path

🤖 Generated with Claude Code

* client: point the stdio deprecation at the first external frame; migrate proxy docs to Path

🤖 Generated with Claude Code