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/mcp-server/src/codex_tool_runner.rs

@@ -363,6 +363,7 @@ async fn run_codex_tool_session_inner(
                     | EventMsg::ExitedReviewMode(_)
                     | EventMsg::RequestUserInput(_)
                     | EventMsg::DynamicToolCallRequest(_)
+                    | EventMsg::DynamicToolCallResponse(_)
                     | EventMsg::SkillRequestApproval(_)
                     | EventMsg::ContextCompacted(_)
                     | EventMsg::ModelReroute(_)