[Server] Stop a deleted session from coming back when an in-flight request saves it

[chr-hertel]

A DELETE while a tool call still runs answers 2xx, but the call's final Session::save() writes the session back - and the id is valid again.

• Session remembers whether the store held it when read or last saved
• if so, save() checks exists() first and returns false instead of recreating a destroyed session
• works for all bundled stores without touching SessionStoreInterface - a tiny window between exists() and write() remains, a tombstone would need a store contract change

Fixes chr-hertel#59

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