fix: dedupe Responses replay item ids

Ensure converted Responses API input items use unique replay ids when restoring assistant messages and function calls. This prevents Codex from rejecting resumed conversations with duplicate rs_* item ids while preserving call_id-based tool result linkage.

nanobot/providers/openai_responses/converters.py

@@ -15,6 +15,7 @@ def convert_messages(messages: list[dict[str, Any]]) -> tuple[str, list[dict[str
     """
     system_prompt = ""
     input_items: list[dict[str, Any]] = []
+    used_item_ids: set[str] = set()
 
     for idx, msg in enumerate(messages):
         role = msg.get("role")
@@ -30,17 +31,19 @@ def convert_messages(messages: list[dict[str, Any]]) -> tuple[str, list[dict[str
 
         if role == "assistant":
             if isinstance(content, str) and content:
+                message_id = _unique_item_id(f"msg_{idx}", used_item_ids)
                 input_items.append({
                     "type": "message", "role": "assistant",
                     "content": [{"type": "output_text", "text": content}],
-                    "status": "completed", "id": f"msg_{idx}",
+                    "status": "completed", "id": message_id,
                 })
             for tool_call in msg.get("tool_calls", []) or []:
                 fn = tool_call.get("function") or {}
                 call_id, item_id = split_tool_call_id(tool_call.get("id"))
+                response_item_id = _unique_item_id(item_id or f"fc_{idx}", used_item_ids)
                 input_items.append({
                     "type": "function_call",
-                    "id": item_id or f"fc_{idx}",
+                    "id": response_item_id,
                     "call_id": call_id or f"call_{idx}",
                     "name": fn.get("name"),
                     "arguments": fn.get("arguments") or "{}",
@@ -97,6 +100,20 @@ def convert_tools(tools: list[dict[str, Any]]) -> list[dict[str, Any]]:
     return converted
 
 
+def _unique_item_id(item_id: str, used: set[str]) -> str:
+    """Return a Responses input item id that is unique within one request."""
+    if item_id not in used:
+        used.add(item_id)
+        return item_id
+
+    suffix = 2
+    while f"{item_id}_{suffix}" in used:
+        suffix += 1
+    unique = f"{item_id}_{suffix}"
+    used.add(unique)
+    return unique
+
+
 def split_tool_call_id(tool_call_id: Any) -> tuple[str, str | None]:
     """Split a compound ``call_id|item_id`` string.