## Why
Extension-contributed tools need to emit visible turn items through
Codex's normal event and persistence pipeline.
## What
- Add `TurnItemEmitter` to extension `ToolCall`s and route the core
implementation through `Session::emit_turn_item_*`.
- Hold weak session and turn references so retained tool calls cannot
keep host state alive.
- Provide a no-op emitter for extension test callers.
## Test Plan
- `just test -p codex-core -E
'test(passes_turn_fields_and_scoped_turn_item_emitter_to_extension_call)'`
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Co-authored-by: jif-oai <jif@openai.com>
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