Implements the server-side handler portion of SEP-2663 for the MCP Spec 2026-07-28 release.

Tracked by umbrella #335. Depends on SEP-2133 extensions framework.

PHP SDK changes

• Register tasks as an extension via the SEP-2133 mechanism; declare server capability and read negotiated state from the request context.
• New src/Server/Handler/Request/TaskGetHandler.php, TaskUpdateHandler.php, TaskCancelHandler.php.
• Register handlers in src/Server/Builder.php (or via extension registry).
• Plumb capability flag through RequestContext so tool authors can detect tasks-eligible requests.

Related

• Umbrella [2026-07-28] Extensions framework + Tasks + MCP Apps #335
• Schema work in a separate issue (Task shape + request/result types)
• Execution model in a separate issue (tool handlers returning task handles)