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/exec/src/lib.rs

@@ -696,6 +696,7 @@ fn should_suppress_agent_job_event(msg: &EventMsg) -> bool {
             | EventMsg::ApplyPatchApprovalRequest(_)
             | EventMsg::RequestUserInput(_)
             | EventMsg::DynamicToolCallRequest(_)
+            | EventMsg::DynamicToolCallResponse(_)
             | EventMsg::ElicitationRequest(_)
             | EventMsg::Error(_)
             | EventMsg::Warning(_)