refactor(llc): extract the message merge algebra into MessageMerging

[VelikovPetar]

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Linear: FLU-724

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Description of the pull request

Moves the pure message/pin/live-location merge and removal operations (formerly private on ChannelClientState) verbatim out of channel.dart into a new unexported static holder MessageMerging under lib/src/core/util/, following the MessageRules precedent. The two message predicates (isShownInChannel and hasValidPin, formerly the top-level _pinIsValid and inline filters) live in a separate unexported MessagePredicates extension on Message.

• Call sites in channel.dart are requalified; the stateful orchestrators and persistence writes stay put.
• The thread orchestrators preserve their exact write/no-write emission behavior via an identical() guard.
• Adds pure unit-test suites (41 tests across message_merging_test.dart and message_predicates_test.dart) covering the merge semantics directly, including previously untested pin-expiry filtering and the thread phantom-guard / empty-thread-pruning rules.
• Zero public API change (lib/stream_chat.dart untouched); no CHANGELOG entry since there is no observable behavior change.

Review tip: git diff --color-moved=dimmed-zebra shows the moved bodies as relocated text.

Test instructions: cd packages/stream_chat && dart test — full package suite passes (1,665 tests, including the existing through-state merge characterization); downstream stream_chat_flutter_core, stream_chat_persistence, and stream_chat_flutter suites verified.

Groundwork for the v11 shared merge-semantics rail.

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[VelikovPetar]

Superseded by #2944.

Closing rather than rebasing: #2930 moves ChannelClientState out of channel.dart, and 99% of this PR's channel.dart diff (281 of 282 changed lines) sits inside that moved block.

#2944 re-hosts the same extraction onto channel_client_state.dart, done as a real three-way merge against #2943 rather than a replay. The two refactors proved disjoint — all nine helpers extracted here still live in channel_client_state.dart after #2943, none having moved into channel_state_mutations.dart. Four conflicts arose, all determinate; the notable one is that #2943 and this PR independently extracted the same shown-in-channel predicate, resolved in favour of this PR's shared MessagePredicates.isShownInChannel.

Public member sets of Channel and ChannelClientState verified identical to #2930 across the whole stack.