[Server] Fix parallel elicitations on one session getting each other's answers

[chr-hertel]

Two tool calls on one session that both elicit got the same request id (1000), so one call resumed with the other one's answer. On top, an elicitation could go out on another call's SSE stream, since all streams drained one session-wide queue.

• requests to the client get a random id (up to 2^53 - 1, JS-safe) instead of a counter read-modify-written in the session
• what a suspended fiber sends is queued per transport in Protocol, so it goes out on the stream of its own call - the session-wide queue stays for messages not tied to a call, like ResourceUpdatedNotification

Overlaps with #580, which also touches the fiber yield handler in Protocol::connect().

Fixes chr-hertel#56

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