[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).

Start by locating TasksExtension, Task::isReadable(), and the proposed TaskStoreInterface entry points. Trace how server-side task state and Task::$inputRequests are handed to the extension, then define the InMemoryTaskStore and Psr16TaskStore responsibilities. Done means save/get/delete work with expiry, bounded oldest-by-last-use eviction exists in memory, and PSR-16 TTL behavior supports PHP-FPM.