After stopping a streaming chat reply, the next message would appear
sent but show no Stop button or "Connecting…" indicator. The cancel
broadcast from the prior generation was leaking into the new SSE
subscription and immediately marking it as errored.
Each `chatManager.sendMessage` now allocates a per-generation id;
`ChatStreamManager` only delivers tagged broadcasts to subscribers
from the matching generation. `sendMessage`'s cleanup also stops
deleting a newer generation's `activeGenerations` slot when an older
one finally unwinds.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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