fix(telegram): avoid conflicting get_updates proxy config

[iodone]

Summary

• avoid combining get_updates_request(...) with get_updates_proxy(...)
• pass the proxy directly into HTTPXRequest(...) for the polling request
• add a regression test covering the proxy-enabled startup path

Why

python-telegram-bot raises a RuntimeError when get_updates_request is already set and get_updates_proxy is then configured on the same builder. This caused TelegramChannel.start() to fail during gateway startup when BUB_TELEGRAM_PROXY was set.

Verification

• uv run pytest tests/test_channels.py -q