fix(gateway): add trusted sender envelope for shared sessions

[xiaoyaner0201]

Summary

• add a verified sender envelope for shared multi-user gateway turns when adapters provide trusted source.user_id / source.user_id_alt
• preserve Slack's <@UID> mention target while extending trusted sender IDs to Discord/Telegram-style shared sessions
• strip forged leading verified-sender envelopes from user text, keep DM/per-user sessions quiet, and fall back to the old unverified display-name prefix when no trusted sender ID is available

Test Plan

• scripts/run_tests.sh tests/gateway/test_image_input_routing_runtime.py tests/gateway/test_active_session_text_merge.py -q
• scripts/run_tests.sh tests/gateway/test_image_input_routing_runtime.py tests/gateway/test_active_session_text_merge.py tests/gateway/test_session.py tests/gateway/test_discord_missed_message_backfill.py tests/gateway/test_telegram_text_batching.py -q
• python -m ruff check gateway/run.py gateway/platforms/base.py tests/gateway/test_active_session_text_merge.py tests/gateway/test_image_input_routing_runtime.py
• python -m compileall -q gateway/run.py gateway/platforms/base.py
• git diff --check

Refs #69961

[alt-glitch]

This was generated by AI during triage.

Related: #13939 is the broader all-chat-context sender-attribution proposal. This PR limits the authenticated envelope and aggregation protection to shared sessions; please choose the desired contract or consolidate the approaches.

[xiaoyaner0201]

Superseded by #69980.

Reason: after this branch advanced past a09e6db230b064d61c7d34b1630bce0e09872f51, the fork ref updated but this PR object did not receive a synchronize event or reflect the new head SHA via the pulls API. I opened #69980 from a fresh branch name so reviewers see the final diff, including the no-trusted-UID forged-envelope regression fix.

[xiaoyaner0201]

Closing in favor of #69980 so the open PR points at the synchronized final branch/head.