feat(ui): add onReactionLongPress reporting the long-pressed reaction

[xsahil03x]

Closes FLU-706.

Adds onReactionLongPress to StreamMessageItem and StreamMessageListView, reporting which reaction was long-pressed so apps can open a reactions bottom sheet showing everyone who reacted.

Mirrors the onReactionTap plumbing from #2852, minus the deprecation machinery — this replaces no existing API, so it is purely additive.

Depends on GetStream/stream-core-flutter#153

stream_core_flutter is pinned to 91a16b2 — the commit on that PR's branch — in melos.yaml, packages/stream_chat_flutter/pubspec.yaml, and docs/docs_screenshots/pubspec.yaml, so CI can resolve it. Re-point all three at a merged SHA once the core PR lands, then merge this.

What changed

• OnReactionLongPress typedef + ReactionLongPressDetails (message + reaction) in typedefs.dart.
• Threaded through StreamMessageListView → StreamMessageItem (+ props and copyWith) → StreamMessageContent → StreamMessageReactions → core StreamReactions.
• The reaction is null for a clustered or overflow chip that maps to no single reaction; an own reaction is reported verbatim, otherwise one is synthesized from the resolver's emoji code — same resolution onReactionTap uses.

No default, on purpose

The reaction chips sit inside the message row's long-press InkWell that opens the actions modal, so a chip-level recognizer wins its hit area. onReactionLongPress therefore has no default arm: left null, null is threaded all the way down, no recognizer is registered, and long-pressing a chip keeps opening the actions modal exactly as before. Existing apps are unaffected whether or not they adopt the callback.

(onReactionTap needs no such care — tap already had a default, opening the reaction detail sheet.)

Testing

• 5 new tests in stream_message_reactions_test.dart: segmented reporting, own-reaction vs. synthesized, overflow → null, cluster → null, and no recognizer when the callback is null.
• 2 new tests in stream_message_item_test.dart pinning the gesture arena outcome on the fully composed item: the chip wins when the callback is set, and the message long press still fires when it is not.
• dart analyze --fatal-infos lib test clean; 1242 tests pass. The 2 stream_message_deleted golden failures are pre-existing on master (verified against a clean checkout).

🤖 Generated with Claude Code

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[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.97%. Comparing base (e75c553) to head (98c57d4).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...er/lib/src/message_widget/stream_message_item.dart	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2903   +/-   ##
=======================================
  Coverage   73.96%   73.97%           
=======================================
  Files         435      435           
  Lines       28149    28158    +9     
=======================================
+ Hits        20821    20830    +9     
  Misses       7328     7328           

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