fix(gateway): avoid false failure reactions on restart cancellation

[bobashopcashier]

Summary

• add a shared ProcessingOutcome lifecycle enum so expected shutdown/restart cancellation is not treated as an ordinary processing failure
• avoid false terminal failure reactions for Discord, Telegram, and Matrix when adapter background tasks are intentionally cancelled during gateway shutdown
• keep genuine delivery failures and genuine exceptions mapped to failure, with regression coverage for both expected and unexpected cancellation paths

What this addresses

Fixes #6315.

Related historical issue: #1414.

Follow-up to PR #1427.

Issue #1414 and PR #1427 fixed an important shutdown bug: old gateway instances could keep unwinding in-flight work after shutdown/replacement because adapter background tasks were not canceled and running agents were not interrupted.

Current main still had a narrower remaining UX bug for reaction-capable adapters:

• gateway/platforms/base.py mapped any asyncio.CancelledError to processing failure
• Discord, Telegram, and Matrix all mapped that failure path to a terminal ❌
• intentional self-restart or shutdown cancellation could therefore surface as a false failure reaction even when the cancellation was expected

This PR distinguishes expected shutdown/restart cancellation from real processing failure so the bot no longer reports a false red ❌ for that case.

Why this is a follow-up to #1427

PR #1427 made gateway shutdown actually cancel adapter background tasks and interrupt running agents.

That exposed the remaining lifecycle classification problem more clearly: once cancellation became deliberate and reliable during shutdown, the shared processing hook still treated that expected cancellation as a generic failure. This PR fixes that semantic gap without regressing genuine failure signaling.

Behavior By Platform

• Discord: remove 👀 on expected cancellation and do not add a terminal ❌
• Telegram: leave the in-progress reaction untouched on expected cancellation instead of replacing it with ❌
• Matrix: do not add a terminal ❌ on expected cancellation
• All three adapters still emit failure reactions for genuine exceptions and genuine delivery failures

Test plan

• .venv/bin/python -m pytest tests/gateway/test_discord_reactions.py tests/gateway/test_telegram_reactions.py tests/gateway/test_base_topic_sessions.py tests/gateway/test_gateway_shutdown.py tests/gateway/test_matrix.py -n0 -q

[bobashopcashier]

Closing to reopen as a refreshed PR that follows the current contributing guide and PR template.

[bobashopcashier]

Superseded by #6942, reopened on a fresh branch with the current contributing-guide/template format.