Recorder cannot save a session to a bare relative filename #8346

Bug

ChatCompletionClientRecorder.finalize() fails when session_file_path is a bare relative filename, such as session.json. os.path.dirname("session.json") is empty, so the unconditional os.makedirs("") raises FileNotFoundError, wrapped as ValueError. Using ./session.json works.

Reproduction

On current main, Windows and Python 3.11.14, with local editable autogen-core, autogen-agentchat and autogen-ext packages and the task-centric memory import dependency installed:

import asyncio
from autogen_core.models import UserMessage
from autogen_ext.models.replay import ReplayChatCompletionClient
from autogen_ext.experimental.task_centric_memory.utils.chat_completion_client_recorder import ChatCompletionClientRecorder

async def main():
    recorder = ChatCompletionClientRecorder(
        ReplayChatCompletionClient(["hello"]),
        mode="record",
        session_file_path="session.json",
    )
    await recorder.create([UserMessage(content="Hi", source="user")])
    recorder.finalize()

asyncio.run(main())

Actual: ValueError: Failed to write records to 'session.json': [WinError 3] The system cannot find the path specified: ''.

Expected: save the recorded interaction to the working directory and allow replaying it. No provider credentials or network inference is required.

I can submit a focused fix that creates a parent directory only when the filename includes one, with record/replay regressions for bare, explicit-relative, nested and absolute paths. #7797 changes text encoding in this module but leaves the directory creation unchanged, so this is a separate failure.

Investigated with Codex assistance; reproduction ran against the real recorder and replay client.