Discover permission requests that are already pending when a thread is
registered with a conversation. Previously, registration only subscribed
to future authorization events, so a waiting tool call could exist in
the thread but remain absent from the conversation's pending-request
lookup and subagent permission counts.

- Seed tracking from existing `WaitingForConfirmation` tool calls in
entry order.
- Use the same deduplicating insertion path for initial state and
subsequent request events, preserving already-tracked order.
- Leave authorization-received removal, conversation-wide shortcut
policy, and preview interaction unchanged.

Not really an issue in practice for the zed agent, but will be
potentially for ACP v2

Release Notes:

- N/A