feat: add async sampling and elicitation tools

Add tools that demonstrate bidirectional MCP tasks where the server
sends requests to the client for async execution:

- trigger-sampling-request-async: Send sampling request with task
  params, client creates task and executes LLM call in background,
  server polls for completion and retrieves result

- trigger-elicitation-request-async: Same pattern for user input,
  useful when user may take time to fill out forms

Both tools:
- Check client capabilities (tasks.requests.sampling/elicitation)
- Accept both CreateTaskResult and direct result responses
- Poll tasks/get for status updates
- Fetch final result via tasks/result

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/everything/docs/features.md

@@ -24,6 +24,8 @@
 - `toggle-subscriber-updates` (tools/toggle-subscriber-updates.ts): Starts or stops simulated resource update notifications for URIs the invoking session has subscribed to.
 - `trigger-sampling-request` (tools/trigger-sampling-request.ts): Issues a `sampling/createMessage` request to the client/LLM using provided `prompt` and optional generation controls; returns the LLM's response payload.
 - `simulate-research-query` (tools/simulate-research-query.ts): Demonstrates MCP Tasks (SEP-1686) with a simulated multi-stage research operation. Accepts `topic` and `ambiguous` parameters. Returns a task that progresses through stages with status updates. If `ambiguous` is true and client supports elicitation, pauses with `input_required` status to gather clarification.
+- `trigger-sampling-request-async` (tools/trigger-sampling-request-async.ts): Demonstrates bidirectional tasks where the server sends a sampling request that the client executes as a background task. Server polls for status and retrieves the LLM result when complete. Requires client to support `tasks.requests.sampling.createMessage`.
+- `trigger-elicitation-request-async` (tools/trigger-elicitation-request-async.ts): Demonstrates bidirectional tasks where the server sends an elicitation request that the client executes as a background task. Server polls while waiting for user input. Requires client to support `tasks.requests.elicitation.create`.
 
 ## Prompts
 
@@ -75,6 +77,26 @@ The server advertises support for MCP Tasks, enabling long-running operations wi
 - `failed`: Task encountered an error
 - `cancelled`: Task was cancelled by client
 
-### Demo Tool
+### Demo Tools
 
+**Server-side tasks (client calls server):**
 Use the `simulate-research-query` tool to exercise the full task lifecycle. Set `ambiguous: true` to trigger the `input_required` flow with elicitation.
+
+**Client-side tasks (server calls client):**
+Use `trigger-sampling-request-async` or `trigger-elicitation-request-async` to demonstrate bidirectional tasks where the server sends requests that the client executes as background tasks. These require the client to advertise `tasks.requests.sampling.createMessage` or `tasks.requests.elicitation.create` capabilities respectively.
+
+### Bidirectional Task Flow
+
+MCP Tasks are bidirectional - both server and client can be task executors:
+
+| Direction | Request Type | Task Executor | Demo Tool |
+|-----------|--------------|---------------|-----------|
+| Client -> Server | `tools/call` | Server | `simulate-research-query` |
+| Server -> Client | `sampling/createMessage` | Client | `trigger-sampling-request-async` |
+| Server -> Client | `elicitation/create` | Client | `trigger-elicitation-request-async` |
+
+For client-side tasks:
+1. Server sends request with task metadata (e.g., `_meta.task.ttl`)
+2. Client creates task and returns `CreateTaskResult` with `taskId`
+3. Server polls `tasks/get` for status updates
+4. When complete, server calls `tasks/result` to retrieve the result