Implements the task persistence layer for SEP-2663 for the MCP Spec 2026-07-28 release.

Tracked by umbrella #335.

PHP SDK changes

• New TaskStoreInterface for persisting task state across requests, mirroring the SessionStoreInterface pattern.
• Default InMemoryTaskStore with eviction limits (mirror InMemorySessionStore's maxSessions: 1000 DoS protection — see CLAUDE.md session security notes).
• FileTaskStore and Psr16TaskStore (PSR-16 cache-backed) following the existing session store ergonomics.
• Wire into Server::builder()->setTaskStore(...).
• Garbage-collection hooks (deterministic GC every N requests, configurable threshold like sessions).

Notes

• Single store interface should cover both server-side task state and the queued server→client requests destined for tasks/update responses.

Related

• Umbrella [2026-07-28] Extensions framework + Tasks + MCP Apps #335
• Companion: tasks/* handlers issue (consumes the store)