fix: keep telegram document failures out of agent input

[sgtworkman]

Summary

• Reply in Telegram for unsupported document types instead of forging gateway error text as agent input.
• Reply in Telegram when document download/cache fails, then stop instead of forwarding an empty/broken event.
• Add regressions for unsupported document rejection and cache/download failure visibility.

Closes #23045

Test Plan

• scripts/run_tests.sh tests/gateway/test_telegram_documents.py -q
• scripts/run_tests.sh tests/gateway/test_telegram_documents.py tests/gateway/test_telegram_photo_interrupts.py tests/gateway/test_telegram_group_gating.py -q

[alt-glitch]

Competing fix with #23061 for #23045 — same Telegram document failure handling.

[teknium1]

Superseded by #53912 (merged: db16854), which fixes the still-live half of #23045 on current main.

Credit to you for the reply-to-user direction — #53912 builds on it. Note the code moved since you opened this: the Telegram adapter is now plugins/platforms/telegram/adapter.py, and the unsupported-document-type rejection your PR also touched no longer exists on main (the handler now accepts any file type, so there's nothing to reject). #53912 lands the cache-failure fix across all five media sites (photo, voice, audio, video, document) and surfaces the failure to both the user and the agent. Thanks!