* refactor(chat): extract streaming state into ChatStreamingController
Moves streaming buffers, streamingMessageId, and the two token/delta
handlers out of ChatViewModel into a dedicated controller. Mirrors the
ChatSearchDelegate seam; _streamingState stays VM-owned and the WS
reducer is untouched. Behavior-preserving, ChatScreen unchanged.
* refactor(chat): drop write-only streamingMessageId in ChatStreamingController
Sourcery flagged streamingMessageId as write-only. Verified via git grep
that it was write-only in origin/main too (zero readers anywhere in the
repo), so removing it is behavior-preserving. beginStreamingMessage() now
delegates its buffer reset to resetStreaming() to remove the duplication
Sourcery noted. Kept the injected scope param — it mirrors the
ChatSearchDelegate seam (that delegate uses scope.launch) and keeps the
controller constructor shape consistent.
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