This PR depends on [#31296](https://github.com/openai/codex/pull/31296)
for the canonical-to-legacy event mappings.
## Description
This PR makes dynamic tools emit canonical `TurnItem::DynamicToolCall`
lifecycle instead of `DynamicToolCallRequest` /
`DynamicToolCallResponse` directly.
App-server v2 now sends the client `DynamicToolCall` request from the
canonical item start. It ignores the mapped legacy request/response
events, so clients receive one item start and one tool request.
## Why
Dynamic tools are a separate migration boundary because their start
event also drives a client request. Keeping that routing change isolated
makes it easier to verify that the request still happens exactly once.
## What changed
- Emit in-progress and completed/failed dynamic tool items from the
dynamic tool handler.
- Move app-server client request dispatch onto canonical dynamic item
starts.
- Add focused app-server coverage for the canonical start notification
and client request.
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