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/tools/src/lib.rs

@@ -62,7 +62,11 @@ pub use responses_api::mcp_tool_to_deferred_responses_api_tool;
 pub use responses_api::mcp_tool_to_responses_api_tool;
 pub use responses_api::tool_definition_to_responses_api_tool;
 pub use tool_call::ConversationHistory;
+pub use tool_call::ExtensionTurnItem;
+pub use tool_call::NoopTurnItemEmitter;
 pub use tool_call::ToolCall;
+pub use tool_call::TurnItemEmissionFuture;
+pub use tool_call::TurnItemEmitter;
 pub use tool_config::ShellCommandBackendConfig;
 pub use tool_config::ToolEnvironmentMode;
 pub use tool_config::ToolUserShellType;