extension-api: add TurnItemEmitter to tool calls (#24813)

## 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)'`

---------

Co-authored-by: jif-oai <jif@openai.com>

codex-rs/ext/extension-api/src/lib.rs

@@ -11,8 +11,10 @@ pub use capabilities::NoopResponseItemInjector;
 pub use capabilities::ResponseItemInjectionFuture;
 pub use capabilities::ResponseItemInjector;
 pub use codex_tools::ConversationHistory;
+pub use codex_tools::ExtensionTurnItem;
 pub use codex_tools::FunctionCallError;
 pub use codex_tools::JsonToolOutput;
+pub use codex_tools::NoopTurnItemEmitter;
 pub use codex_tools::ResponsesApiTool;
 pub use codex_tools::ToolCall;
 pub use codex_tools::ToolExecutor;
@@ -20,6 +22,8 @@ pub use codex_tools::ToolName;
 pub use codex_tools::ToolOutput;
 pub use codex_tools::ToolPayload;
 pub use codex_tools::ToolSpec;
+pub use codex_tools::TurnItemEmissionFuture;
+pub use codex_tools::TurnItemEmitter;
 pub use codex_tools::parse_tool_input_schema;
 pub use codex_tools::parse_tool_input_schema_without_compaction;
 pub use contributors::ApprovalReviewContributor;