session/resume idempotency

[LucaDiba]

Claude and Codex allow calling session/load multiple times on the same session. GitHub Copilot errors with Session {} is already loaded.

Streamable HTTP & WebSocket Transport mentions that "A session persists on the server independently of any one connection, so after a dropped connection a client can reconnect and resume it via session/load."

But I couldn't find any reference to this in the docs outside of this sentence in the RFD.

Should we add something like "session/resume MUST be idempotent" in the v2 (and v1? even though this is not the case today) Session Setup page?