refactor: centralize gateway module and address review fixes

Move the gateway runtime files into a dedicated core/gateway module and fix follow-up issues in session deletion, history import batching, message IDs, and legacy thread parsing.

Fixes #253

Co-authored-by: Codex <noreply@openai.com>

packages/coding-agent/src/core/gateway/helpers.ts

@@ -0,0 +1,29 @@
+import type { AgentSession } from "../agent-session.js";
+
+export function extractMessageText(message: { content: unknown }): string {
+  if (!Array.isArray(message.content)) {
+    return "";
+  }
+  return message.content
+    .filter((part): part is { type: "text"; text: string } => {
+      return (
+        typeof part === "object" &&
+        part !== null &&
+        "type" in part &&
+        "text" in part &&
+        part.type === "text"
+      );
+    })
+    .map((part) => part.text)
+    .join("");
+}
+
+export function getLastAssistantText(session: AgentSession): string {
+  for (let index = session.messages.length - 1; index >= 0; index--) {
+    const message = session.messages[index];
+    if (message.role === "assistant") {
+      return extractMessageText(message);
+    }
+  }
+  return "";
+}