feat: report a subagent as requires_action while its request is open for subagents RFD clients (#1322)

The third of three stacked PRs that replace #1257. It's based on #1321,
so its diff shows only the `requires_action` part. Merge #1320 and #1321
first; this PR then retargets to `main`.

1. #1320: the RFD form for a v1 client that declares `subagents` and
isn't AIR.
2. #1321: a child keeps its session across SendMessage resumes.
3. **This PR:** `requires_action` while a child's request is open.

**AIR is unchanged.**

## What changes

The [subagents
RFD](https://github.com/agentclientprotocol/agent-client-protocol/blob/main/docs/rfds/subagents.mdx)
reports a child that waits for the user as `requires_action`.

- **While a request is open:** a child whose permission or question
request is open reports `requires_action` on its parent, then `running`
again when its last open request ends
(`NativeSubagentRuntime.awaitingUser`). The agent wraps the permission
and elicitation requests of a child session in it
(`withChildAwaitingUser`).
- **Overlapping requests** keep `requires_action` until the last one
ends. A child that ended while it waited doesn't report `running` again.
- **A resume while a request is open** reports `requires_action`, not
`running`.

## Live run

Against real Claude Code (Sonnet), a child's Bash permission request
showed `running` → `requires_action` while the request was open →
`running` once it was answered. No schema violations.

## Follow-ups (low severity, not in this PR)

- **Brief `running` after a withdrawal:** after a cancel withdraws a
child's request, the child briefly reports `running` before the SDK's
stop event. The same happens at a stream end, whose cleanup aborts every
pending request.
- **Ignored cancel requests:** AskUserQuestion's `createElicitation` is
not raced against the abort. If a client ignores `$/cancel_request`, the
child's open-request count stays up, and a later resume reports
`requires_action`.

## Tests

- **End to end:** `requires_action` around a child's permission request.
The cancel test now also expects the background child's
`requires_action` before the cancel.
- **Runtime:** overlapping requests, a child that ended while it waited,
and a resume with an open request.
- **Checks:** `npm run check`, build and all 2877 tests pass on this
branch, whose tree is #1257's with `main` merged in. CI runs only on PRs
to `main`, so I ran `ci.yml` on this branch by hand.

README.md

@@ -41,10 +41,11 @@ A client that declares `clientCapabilities.subagents`, other than JetBrains AIR,
 Task subagent as its own child session, following the [subagents RFD](https://github.com/agentclientprotocol/agent-client-protocol/blob/main/docs/rfds/subagents.mdx):
 
 - `subagent_update` on the parent announces the child before any of its traffic, with the
-  parent's `title` and `description` for it, and reports its work state: `running`, and `idle`
-  when its work ends, with `end_turn` or `cancelled` when the SDK gives the reason. A failure is
-  `idle` with the `error` stop reason and the SDK's error text as its `error`. The child keeps its
-  session for every delegation to it.
+  parent's `title` and `description` for it, and reports its work state: `running`,
+  `requires_action` while one of its requests is open, and `idle` when its work ends, with
+  `end_turn` or `cancelled` when the SDK gives the reason. A failure is `idle` with the `error`
+  stop reason and the SDK's error text as its `error`. The child keeps its session for every
+  delegation to it.
 - The prompt of the Agent or Task call that launches the child, and the message of each
   SendMessage call that resumes it, is a `session_message` in the child's transcript. The tool
   call stays a tool call of the session that made it.