[fix] use thread-local aiohttp.ClientSession, as it is not thread-safe

[utterstep]

Description

Fix thread-safety issue in async SessionManager.

SessionManager in telebot/asyncio_helper.py is currently a module-level singleton. Its session attribute (which is an aiohttp.ClientSession) was stored as a plain instance variable, meaning it was shared across all threads => when multiple threads each run their own event loop with separate AsyncTeleBot instances, they all use the same aiohttp.ClientSession — which is not thread-safe.

I encountered this issue myself – were getting unpredictable 2026-02-26 23:26:15,914 (asyncio_helper.py:103 workflow-exec_1) ERROR - TeleBot: "Unknown error: RuntimeError" errors in separate threads, each with its own eventloop and AsyncTeleBot instance local to this loop.

This PR wraps the session attribute in threading.local(), so each thread gets its own aiohttp.ClientSession, matching how the sync side already handles this via util.per_thread.

Describe your tests

Confirmed that multiple AsyncTeleBot instances running on separate threads with separate event loops no longer share aiohttp session state across threads.

Python version: 3.12

OS: macOS

Checklist:

• I added/edited example on new feature/change (if exists) n/a, see below
• My changes won't break backward compatibility
• I made changes both for sync and async n/a, see below

No examples needed — this is an internal bugfix with no external API changes.

The sync side (apihelper.py) already uses util.per_thread for thread-local sessions, so no sync changes were needed.

[Badiboy]

@utterstep Thank you.

@coder2020official LGTM.