Expose conversation history to tool lifecycle extensions (#38403)

## What changed

- Add a shared, read-only `ConversationHistorySnapshot` capability to the extension API.
- Include the snapshot in `ToolStartInput`, preserving conversation order while excluding contextual user messages.
- Avoid acquiring a history snapshot when no tool lifecycle contributors are registered.

## Testing

- Verify snapshots share existing response items, remain stable as history changes, and filter contextual user messages.
- Verify tool-start callbacks receive the history available for successive tool calls.

GitOrigin-RevId: 822cae33781dd876ea628f104be08d8a449a4900

codex-rs/core/src/session/mod.rs

@@ -56,6 +56,7 @@ use codex_connectors::connector_runtime_context_key;
 use codex_exec_server::Environment;
 use codex_exec_server::EnvironmentManager;
 use codex_execpolicy::prefix_rule_migration;
+use codex_extension_api::ConversationHistorySnapshot;
 use codex_extension_api::ExtensionDataInit;
 use codex_extension_api::LoadedUserInstructions;
 use codex_extension_api::PromptFragment;
@@ -3668,6 +3669,13 @@ impl Session {
         state.clone_history()
     }
 
+    pub(crate) async fn conversation_history_snapshot(
+        &self,
+    ) -> Arc<dyn ConversationHistorySnapshot> {
+        let state = self.state.lock().await;
+        state.history.conversation_history_snapshot()
+    }
+
     pub(crate) async fn current_window_id(&self) -> String {
         let state = self.state.lock().await;
         let thread_id = self.thread_id;