fix: Telegram proxy should be opt-in, not auto-discovered

[Ehco1996]

Problem

Currently, resolve_proxy() automatically discovers and uses proxies in this order:

1. Explicit BUB_TELEGRAM_PROXY config
2. Environment variables (HTTPS_PROXY, http_proxy, etc.)
3. macOS system proxy (via scutil --proxy)
4. No proxy

This is dangerous. Steps 2 and 3 cause "accidental proxying" - the bot may silently use a proxy that the user never intended to use.

Real-world scenario

User has a transparent proxy on their router. curl and browsers work fine without any configuration. But python-telegram-bot (using httpx) does NOT automatically use transparent proxies.

However, if the user happens to have HTTPS_PROXY set for some other tool, Bub will automatically pick it up and use it - this is unexpected and wrong.

Expected behavior

Configuration	Behavior
BUB_TELEGRAM_PROXY not set	Direct connection, no proxy
BUB_TELEGRAM_PROXY=http://...	Use explicitly configured proxy
HTTPS_PROXY (env var)	Ignore (unless explicitly enabled)
macOS system proxy	Ignore (unless explicitly enabled)

Suggested fix

Modify resolve_proxy() in src/bub/channels/utils.py:

def resolve_proxy(explicit_proxy: str | None) -> tuple[str | None, str]:
    if explicit_proxy:
        return explicit_proxy, "explicit"
    # REMOVE: auto-discovery of env vars and system proxy
    # Only use proxy if explicitly configured
    return None, "none"

If users want to use environment variables, they can explicitly opt-in:

# Option 1: Explicit config
export BUB_TELEGRAM_PROXY="$HTTPS_PROXY"
bub message

# Option 2: Direct parameter (if supported)
bub message --telegram-proxy "$HTTPS_PROXY"

Impact

• Security: Prevents accidental proxy usage
• Predictability: Bot behaves the same regardless of user environment
• Transparency: User must explicitly choose to use a proxy

---

Related: The current behavior caused confusion in a real deployment where a user with a transparent router proxy expected the bot to "just work" without any proxy configuration, but the bot was trying (and failing) to auto-discover proxies.

[Ehco1996]

this iusse is generated by bub :)

[yihong0618]

can you fix it also use bub?