fix(llc, core, ui, localization): hide raw StreamChannel errors behind themed, connection-aware states

[xsahil03x]

FLU-628

Port of #2846 to v9.

Raw StreamChannel initialization errors — and the default list/scroll-view error states — were leaking unthemed, unlocalized error details to end users. This routes them through themed, localized, connection-aware default states (no internet / slow connection / generic), overridable at every level.

What changed

stream_chat

• Add StreamChatNetworkError.type (StreamChatNetworkErrorType) exposing the transport failure kind; deprecate isRequestCancelledError in favor of type == StreamChatNetworkErrorType.cancel.
• A failed channel initialization no longer leaves Channel.initialized errored or its name/image/extraData setters throwing — a subsequent successful (re)init recovers cleanly (supports the retry flow).
• Clearer toString() for StreamWebSocketError / StreamChatNetworkError.

stream_chat_flutter_core

• Add StreamChannelState.retry() to re-run a failed channel init.
• Add DefaultStreamChannelBuilders, an inherited widget supplying default loading/error builders to descendant StreamChannels.
• The default StreamChannel error state no longer exposes raw error details.

stream_chat_flutter

• StreamChat now installs themed, connection-aware default loading & error states for StreamChannel — overridable per-StreamChannel or via DefaultStreamChannelBuilders.
• Default list/scroll-view error states (channel, message, member, user, thread, poll-vote, search, photo) are now connection-aware, falling back to each view's specific error text.
• StreamScrollViewErrorWidget gains errorSubtitle (+ style), defaulting to the generic localized copy.

stream_chat_localizations

• New connection-error strings (connectionError*, slowConnectionError*, genericError*) for all supported locales.

Port notes — differences from #2846

The original PR targets the current master (v10) baseline. This port adapts it to v9:

• Dart 3.6.2 (v9) vs 3.11 (master): dot-shorthands (.unknown, .cancel, .w600, .symmetric, …) are rewritten as explicit enum/const references, and StreamChatError keeps EquatableMixin (v9's deliberate choice).
• Design system: the original UI targets v10's new design system (StreamButton, StreamLoadingSpinner, context.streamColorScheme, streamSpacing, icons.*, stream_core_flutter), none of which exist on v9. The scroll-view error widget, the StreamChat default channel loading/error states, and the message-list error state are re-implemented with v9's chatThemeData / ElevatedButton / CircularProgressIndicator / Icons.*.
• message_list_view: v9's error state is a single styled Text, so it becomes connection-aware via resolveNetworkErrorText(...).title (no subtitle/retry button, matching v9's existing UI).
• All list-view retry buttons use v9's existing "Retry" default (kept consistent across views); the full-screen StreamChannel error uses "Try Again".

Intentionally omitted (v10-only):

• The StreamScrollViewLoadingWidget size / StreamLoadingSpinnerSize redesign (depends on v10-only stream_core_flutter); v9 keeps CircularProgressIndicator + height/width.
• StreamReactionListView — a v10-only component that doesn't exist on v9.
• The incidental dio version bump.

Testing

• All four touched package suites pass; dart analyze --fatal-infos clean across all four packages.
• New tests cover the default channel error/loading states, the retry-clears-stale-error flow, and full localization coverage.
• The only failing tests locally are pre-existing golden mismatches in unrelated components (voice-recording attachments, list tiles) — local-platform (macOS) noise; CI rasterizes against goldens/ci/, and none of the touched files have golden tests.

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[codecov]

Codecov Report

❌ Patch coverage is 57.41758% with 155 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (v9@b861588). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...c/scroll_view/stream_scroll_view_empty_widget.dart	0.00%	12 Missing ⚠️
.../channel_scroll_view/stream_channel_grid_view.dart	0.00%	10 Missing ⚠️
.../channel_scroll_view/stream_channel_list_view.dart	0.00%	10 Missing ⚠️
...view/draft_scroll_view/stream_draft_list_view.dart	0.00%	10 Missing ⚠️
...ew/member_scroll_view/stream_member_grid_view.dart	0.00%	10 Missing ⚠️
...ew/member_scroll_view/stream_member_list_view.dart	0.00%	10 Missing ⚠️
...h_scroll_view/stream_message_search_grid_view.dart	0.00%	10 Missing ⚠️
...h_scroll_view/stream_message_search_list_view.dart	0.00%	10 Missing ⚠️
...l_vote_scroll_view/stream_poll_vote_list_view.dart	0.00%	10 Missing ⚠️
...ew/thread_scroll_view/stream_thread_list_view.dart	0.00%	10 Missing ⚠️
... and 9 more

Additional details and impacted files

@@          Coverage Diff          @@
##             v9    #2849   +/-   ##
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  Coverage      ?   67.32%           
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  Files         ?      431           
  Lines         ?    27370           
  Branches      ?        0           
=====================================
  Hits          ?    18427           
  Misses        ?     8943           
  Partials      ?        0           

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