[Chore] Streaming state guard-then-copy pattern duplicated 5x in chat_notifier.dart

[ASahu16]

Problem

The guard-then-copy state-update pattern

if (state.valueOrNull != null) {
  state = AsyncData(state.requireValue.copyWith(...));
}

is copy-pasted 5 times across _runAgentTurn/send() in lib/features/chat/presentation/chat_notifier.dart (around line 566) instead of factored into one helper.

Why this matters

A future change to how streaming state is updated (e.g. handling a race where state briefly errors, or adding a new field) requires editing 5 call sites by hand; missing one produces an inconsistent state update on just that code path with no compiler error to catch it.

Found during v1.2.0 release-readiness code review of the main...develop diff.

Fix direction

Factor the pattern into a small _updateState(ChatState Function(ChatState) update) helper used at all 5 call sites.