When the user runs /stop or a session is interrupted mid-flight, the
👀 in-progress reaction lingered on the user's message indefinitely.
Without another agent run to swap it for 👍/👎, the eyes stayed there
forever — visually misleading (looks like the agent is still working).
Fix: on ProcessingOutcome.CANCELLED, call set_message_reaction with
reaction=None to clear all reactions on the message. Documented Bot API
semantics (equivalent to Bot API 10.0's deleteMessageReaction, but works
on PTB 22.6 already without the version bump).
Test changes:
- Renamed test_on_processing_complete_cancelled_keeps_existing_reaction
→ test_on_processing_complete_cancelled_clears_reaction; updated
assertion to expect set_message_reaction(reaction=None).
- Added test_on_processing_complete_cancelled_skipped_when_disabled
(TELEGRAM_REACTIONS=false short-circuits).
- Added test_clear_reactions_handles_api_error_gracefully and
test_clear_reactions_returns_false_without_bot to cover the new
_clear_reactions helper.
Telegram's Bot API only allows a specific set of emoji for bot reactions
(the ReactionEmoji enum). ✅ (U+2705) and ❌ (U+274C) are not in that
set, causing on_processing_complete reactions to silently fail with
REACTION_INVALID (caught at debug log level).
Replace with 👍 (U+1F44D) / 👎 (U+1F44E) which are always available in
Telegram's allowed reaction list. The 👀 (eyes) reaction used by
on_processing_start was already valid.
Based on the fix by @ppdng in PR #6685.
Fixes#6068
