Implement TypingCapable interface for LINE channel using the
loading animation API (1:1 chats only, no group support).
- Add StartTyping() with 50s periodic refresh and context-based stop
- Integrate PlaceholderRecorder.RecordTypingStop in processEvent
- Skip RecordPlaceholder (LINE has no message edit API)
- Change sendLoading to accept context and return error
- Relax callAPI status check from 200 to 2xx range
Design consulted with Codex (GPT-5.2).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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