[Server] SEP-2663 Tasks: TasksExtension, capability gate, tasks/* handlers

Server-side handler portion of SEP-2663 for the 2026-07-28 release. Tracked by umbrella #335; sits on the SEP-2133 extensions framework (#344).

Scope

• Mcp\Server\Task\TasksExtension implements MethodProvidingExtensionInterface and ArgumentProvidingExtensionInterface (new, generic: an extension registers its message classes, handlers and injectable handler arguments through Builder::enableExtension(); the core never names the extension). Enable with new TasksExtension($store, $inputHandler) and the tasks/get / tasks/update / tasks/cancel surface appears; no separate setTaskStore() builder knob.
• Handlers under Mcp\Server\Task\Handler\: TasksGetHandler, TasksUpdateHandler, TasksCancelHandler.
• Capability gate: TaskCapabilityGuard reads what initialize left in the session and refuses the tasks/* methods with -32021 (missing required client capability) for a client that did not declare the extension.
• Handlers get a TaskContext by declaring the parameter: isSupported() so a handler that can work either way asks first, create() stores the task before returning the CreateTaskResult (so the first tasks/get cannot arrive before the task exists) and refuses an undeclared client with -32021, getStore().

Start with the SEP-2133 extensions framework in #344 and the Builder::enableExtension() path, then inspect TasksExtension and the handlers under Mcp\Server\Task\Handler. Trace TaskCapabilityGuard and TaskContext to verify the declared-client checks, task creation ordering, and store access. Done means the tasks/* surface is registered through TasksExtension and missing client capability returns -32021 as specified.