feat: explain interrupted AI responses with message tips (#1344)

* feat: explain interrupted AI responses with message tips

* fix: shorten length guidance and verify provider stop reasons

* style: show finish reason tips as an icon-led muted note

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UAo4M2J4wArf5JdmL7D4j4

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
(cherry picked from commit 1e499e560ff06cb4341ffcfec8ad8f6650f92e9c)

(cherry picked from pro commit ef0cc6fd51fba1c7b189ba6edfd36924219b80ef)