[Server] SEP-2663 Tasks: TaskStoreInterface with InMemory and PSR-16 stores

Task persistence for SEP-2663 for the 2026-07-28 release. Tracked by umbrella #335.

Scope

• Mcp\Server\Task\TaskStoreInterface: save(), get() (null for unknown or expired — a client cannot tell them apart and does not need to), delete().
• InMemoryTaskStore with a task limit (default 1000, oldest-by-last-use evicted) so unbounded task creation cannot exhaust memory. Right for stdio and single-process runtimes; wrong under PHP-FPM, where the worker that creates a task is not the one polled for it.
• Psr16TaskStore (PSR-16 cache-backed) for PHP-FPM — a filesystem adapter covers the "file store" case, so no separate FileTaskStore.
• Expiry is TTL-based (Task::isReadable() on read, cache TTL for PSR-16) plus the in-memory limit; no request-counted GC pass.
• The store is the one place both server-side task state and the pending server→client asks (Task::$inputRequests) live.
• The store is handed to TasksExtension, not to the builder (see #346).

Inizia individuando TasksExtension, Task::isReadable() e i punti di ingresso proposti di TaskStoreInterface. Traccia il modo in cui lo stato delle attività lato server e Task::$inputRequests vengono passati all’estensione, quindi definisci le responsabilità di InMemoryTaskStore e Psr16TaskStore. Il lavoro è completato quando save/get/delete funzionano con la scadenza, in memoria esiste un’evizione limitata dell’elemento più vecchio in base all’ultimo utilizzo e il comportamento TTL di PSR-16 supporta PHP-FPM.