feat(telegram): add dedicated TELEGRAM_PROXY env var and config.yaml proxy_url support

Pass platform_env_var="TELEGRAM_PROXY" to resolve_proxy_url() in both
telegram.py (main connect) and telegram_network.py (fallback transport),
so a Telegram-specific proxy takes priority over the generic HTTPS_PROXY.

Also bridge telegram.proxy_url from config.yaml to the TELEGRAM_PROXY
env var (env var takes precedence if both are set), add OPTIONAL_ENV_VARS
entry, docs, and tests.

Composite salvage of four community PRs:
- Core approach (both call sites): #9414 by @leeyang1990
- config.yaml bridging + docs: #6530 by @WhiteWorld
- Naming convention: #9074 by @brantzh6
- Earlier proxy work: #7786 by @ten-ltw

Closes #9414, closes #9074, closes #7786, closes #6530

Co-authored-by: WhiteWorld <WhiteWorld@users.noreply.github.com>
Co-authored-by: brantzh6 <brantzh6@users.noreply.github.com>
Co-authored-by: ten-ltw <ten-ltw@users.noreply.github.com>

website/docs/reference/environment-variables.md

@@ -170,6 +170,7 @@ For cloud sandbox backends, persistence is filesystem-oriented. `TERMINAL_LIFETI
 | `TELEGRAM_WEBHOOK_SECRET` | Secret token for verifying updates come from Telegram |
 | `TELEGRAM_REACTIONS` | Enable emoji reactions on messages during processing (default: `false`) |
 | `TELEGRAM_IGNORED_THREADS` | Comma-separated Telegram forum topic/thread IDs where the bot never responds |
+| `TELEGRAM_PROXY` | Proxy URL for Telegram connections — overrides `HTTPS_PROXY`. Supports `http://`, `https://`, `socks5://` |
 | `DISCORD_BOT_TOKEN` | Discord bot token |
 | `DISCORD_ALLOWED_USERS` | Comma-separated Discord user IDs allowed to use the bot |
 | `DISCORD_HOME_CHANNEL` | Default Discord channel for cron delivery |