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

[VelikovPetar]

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Linear: FLU-724

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• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

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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No UI changes.

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