fix(telegram): allow disabling typing indicators

[kenners22]

Summary

• Bridge telegram.typing_indicators from config into Telegram adapter extra config
• Allow typing indicators to be disabled by config or TELEGRAM_TYPING_INDICATORS env override
• Add focused regression coverage

Test Plan

• python -m py_compile gateway/config.py gateway/platforms/telegram.py tests/gateway/test_telegram_typing_indicator_config.py
• python -m pytest tests/gateway/test_telegram_typing_indicator_config.py -q -o addopts=

[teknium1]

Superseded by #55394 (merged to main, commit 05ac167). That PR adds a generic per-platform typing_indicator flag at the shared _keep_typing spawn site, so the Telegram disable you wanted now works via typing_indicator: false under your telegram platform config — same key for every platform. Thanks for the contribution — credited in the merged PR.