feat(app-server): add ThreadItem::DynamicToolCall (#12732)

Previously, clients would call `thread/start` with dynamic_tools set,
and when a model invokes a dynamic tool, it would just make the
server->client `item/tool/call` request and wait for the client's
response to complete the tool call. This works, but it doesn't have an
`item/started` or `item/completed` event.

Now we are doing this:
- [new] emit `item/started` with `DynamicToolCall` populated with the
call arguments
- send an `item/tool/call` server request
- [new] once the client responds, emit `item/completed` with
`DynamicToolCall` populated with the response.

Also, with `persistExtendedHistory: true`, dynamic tool calls are now
reconstructable in `thread/read` and `thread/resume` as
`ThreadItem::DynamicToolCall`.

codex-rs/app-server/README.md

@@ -724,6 +724,13 @@ When a dynamic tool is invoked during a turn, the server sends an `item/tool/cal
 }
 ```
 
+The server also emits item lifecycle notifications around the request:
+
+1. `item/started` with `item.type = "dynamicToolCall"`, `status = "inProgress"`, plus `tool` and `arguments`.
+2. `item/tool/call` request.
+3. Client response.
+4. `item/completed` with `item.type = "dynamicToolCall"`, final `status`, and the returned `contentItems`/`success`.
+
 The client must respond with content items. Use `inputText` for text and `inputImage` for image URLs/data URLs:
 
 ```json