docs(examples): ext-tasks — a minimal in-process Tasks server and client

[mattzcarey]

Draft, third of a stack on #2782 (Tasks extension), which sits on #2820 (extensions). Review the last commit only.

What this adds

examples/ext-tasks: a minimal server and client for the Tasks extension, everything in-process — no queue, no workflow engine, no database. It is the smallest thing that exercises the whole wire.

Server. TasksExtension over an InMemoryTaskStore, installed with new McpServer(info, { extensions: [tasks] }). One tool, bake_cake, calls tasks.create(ctx) and answers with the task handle at once; the work is a plain async function driven by timers that reports progress through handle.status, asks the client which frosting to use through handle.requireInput (input_required), and honours cancellation between steps through handle.signal.

Client. TasksClientExtension installed on the Client. It calls the tool, follows the task with waitFor — answering the frosting question through tasks.update inside onUpdate — and asserts the completed result. It then starts a second task and cancels it while it waits for input, and checks an unknown task id answers -32602.

Both legs pass locally over stdio and Streamable HTTP; modern era only, since task handles and tasks/* ride the 2026-07-28 per-request capabilities envelope.

[client] task … created: working
[client] saw: working (mixing 3 layers) -> input_required (mixing 3 layers) -> completed (baking with chocolate frosting)
[client] task … ended: cancelled

Also in this PR

• examples/tsconfig.json maps the two ext/tasks package subpaths (and the shared core-internal/ext/tasks wire module) to source for typechecking, like the existing subpath entries.
• A row in examples/README.md.

Requires #2782 and #2820.

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