fix: Telegram require_mention gating in groups

[vrknetha]

Summary

• enforce strict Telegram group gating when require_mention is enabled
• support targeted Telegram command syntax /command@botusername in strict mode
• keep DMs unaffected and preserve mention/reply/prefix trigger behavior
• bridge Telegram config values into gateway runtime with platforms.telegram.extra override precedence

Tests

• tests/gateway/test_telegram_require_mention.py
• tests/gateway/test_telegram_group_gating.py
• tests/gateway/test_config.py
• tests/gateway/test_telegram*.py