fix(llc, core): port e2e-suite thread fix and disposed-channel sort crash to v9

[xsahil03x]

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Linear: FLU-668, FLU-692

Github Issue: #

CLA

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• The code changes follow best practices
• Code changes are tested

Description of the pull request

Ports two master PRs to v9 on one branch:

• #2874 (66ae984f8) — "fix(llc, ui): Issues found by the e2e suites"
• #2887 (cb43fc417) — "fix(core): sort channels with disposed state last instead of crashing"

Each fix was checked against v9 rather than ported blindly:

Fix	v9	Ported
FLU-668 — getReplies adds the parent to the thread's replies	has the bug	✅
FLU-692 — local sort null-asserts on a disposed channel	has the bug	✅
FLU-686 — channel-list row previews another channel's message	has the bug	❌ master only, by choice
FLU-669 — thread-reply label reads "1 replies", unlocalized	not affected	❌ n/a

FLU-668 — parent message kept out of thread replies

v9's updateThreadInfo is structured differently from master's (no _mergeMessagesIntoExisting; two paths converging on messagesToMerge), so this is hand-applied rather than cherry-picked. The parent is filtered out before the upsert: false branch narrows to already-loaded replies, so that branch's isEmpty early-return still guards against creating a thread entry from nothing.

FLU-692 — disposed channels sort last

Clean cherry-pick of cb43fc417; v9 has the identical (it) => it.state!.channelState null-assert. Conflicts were indentation-only (v9's 80-col short style) plus the changelog.

Not ported

FLU-686 (_ChannelLastMessageTextState preview-cache guards) — v9 does have this bug, and the guards were prepared and verified here, but we're deliberately keeping the fix on master only. stream_chat_flutter in this PR is byte-identical to v9.

FLU-669 genuinely does not apply: v9 already reads the label from context.translations.threadReplyCountText in message_widget/bottom_row.dart. The hardcoded '$replyCount replies' that #2874 fixes lives in message_widget/stream_message_item.dart, a file that only exists on master — it came in with the design refresh.

Testing

Verified with v9's pinned toolchain (Flutter 3.27.4 / Dart 3.6.2 per .fvmrc), not a newer local SDK — a newer one restyles whole files.

Both fixes were mutation-checked by reverting the production change and confirming the matching test fails:

Fix	Test	Without the fix
FLU-668	`.getReplies` keeps the parent message out of the thread	fails: Expected: <3> Actual: <4>
FLU-692	local sort places channels with disposed state last…	fails: _TypeError: Null check operator used on a null value

stream_chat              1307 passed / 2 skipped   analyze clean   format clean
stream_chat_flutter_core   20 passed (controller)   analyze clean   format clean

stream_chat is at 1307 rather than the 1303 this branch produced before the latest v9 merge — the merge brought in the four mark-read tests from #2891, which landed on v9 in the meantime.

Screenshots / Videos

No UI changes — stream_chat_flutter is untouched relative to v9.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (v9@e7e4ebe). Learn more about missing BASE report.

Additional details and impacted files

@@          Coverage Diff          @@
##             v9    #2893   +/-   ##
=====================================
  Coverage      ?   67.30%           
=====================================
  Files         ?      431           
  Lines         ?    27393           
  Branches      ?        0           
=====================================
  Hits          ?    18438           
  Misses        ?     8955           
  Partials      ?        0           

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