refactor(llc, ui)!: adopt stream_core's platform detector and sealed exception family

[xsahil03x]

Linear: FLU-

CLA

• I have signed the Stream CLA (required).
• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

🎯 Goal

Closes the migration: the platform detector goes, and the old error tree is deleted outright rather than left deprecated.

🛠 Implementation details

• CurrentPlatform and PlatformType become core's, re-exported from this package. CurrentPlatform.name becomes .operatingSystem, reporting the same string. Deleting our copy also drops the js_interop branch chat needed only because its own stub threw.
• StreamChatError, StreamChatNetworkError, StreamChatNetworkErrorType and StreamWebSocketError are removed, not deprecated. Nothing throws them any more, so a deprecated type would leave on StreamChatNetworkError catch compiling while silently matching nothing — a break you ship without noticing. Removing it is a compile error instead.
• isRetriable becomes an extension on StreamChatException, so it reads the same on any failure.
• stream_chat_flutter's three attachment-validation errors move to their own sealed AttachmentValidationError family. They are returned, never thrown, so typing a refused attachment as a transport failure invited callers to treat it as one.
• Adds a test pinning the interceptor pipeline order. The existing tests only asserted each interceptor was present, which a fully reordered pipeline satisfies too.

☑️ Verification

melos run analyze, melos run format and melos run test:dart pass against the pinned stream_core commit this stack resolves from.

The pipeline-order test was checked to fail on a swap rather than only to pass as written — moving ApiErrorInterceptor after the logging interceptor makes it red.

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[codecov]

Codecov Report

❌ Patch coverage is 56.09756% with 18 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (migrate-core/05-coordinate-listext@c656354). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...es/stream_chat/lib/src/client/channel/channel.dart	36.84%	12 Missing ⚠️
packages/stream_chat/lib/src/client/client.dart	70.00%	3 Missing ⚠️
packages/stream_chat/lib/src/ws/websocket.dart	57.14%	3 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                          @@
##             migrate-core/05-coordinate-listext    #2960   +/-   ##
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  Coverage                                      ?   74.03%           
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  Files                                         ?      424           
  Lines                                         ?    28364           
  Branches                                      ?        0           
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  Hits                                          ?    20999           
  Misses                                        ?     7365           
  Partials                                      ?        0           

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[VelikovPetar]

I am curious why is this no longer a Stream- scoped error?

Is it because it indicates an invalid use of the API, and not really a client-level error?

Edit: I saw the updated changelog - it answers my question! We would probably need the sweep the other API surface for similar stuff.

[xsahil03x]

Right on both counts — StateError because it reports a caller mistake (cancelling an upload that never started, or was already cancelled), not something the server said.

On the sweep: it is already done across the stack. There is no throw StreamChatError left anywhere in packages/*/lib, and the split now reads:

package	StateError	ArgumentError	StreamClientException
stream_chat	15	11	3
stream_chat_flutter_core	1	0	0
stream_chat_flutter	2	4	0
stream_chat_persistence	1	2	0

The rule applied was: ArgumentError for a bad argument, StateError for a call made at the wrong time, and StreamClientException only where the SDK itself failed at something it promised to do — the three in stream_chat are all the attachment-upload aggregate (_uploadFailureMessage), which is a real SDK failure rather than misuse. ERROR_LAYER.md is the source for that boundary.