Add userClaim field across chat entities

[AndyTWF]

Summary

• Add server-provided userClaim (JWT claim) field to Message, RoomReaction, MessageReaction, TypingSetEvent.Change, and PresenceMember interfaces
• Extract userClaim from realtime message/annotation extras for Messages, RoomReactions, and MessageReactions
• Parse userClaim from REST API responses for Messages
• Cache userClaim across typing heartbeats and preserve in timed-stop events
• Define userClaim API on PresenceMember (always null pending ably-java PresenceMessage.extras support)
• Add MessageExtras?.userClaim() helper for safe extraction from extras JSON

Spec: CHA-M2h, CHA-MR7d, CHA-ER2a, CHA-PR6g, CHA-T13a1

Ref: ably/specification#423, ably/ably-chat-js#711

Test plan

• Unit tests for userClaim parsing from realtime message extras (present, absent, non-string)
• Unit tests for userClaim parsing from REST API responses
• Unit tests for userClaim extraction from room reaction extras
• Unit tests for userClaim extraction from annotation extras in raw message reaction events
• Unit tests for userClaim in typing start/stop events, heartbeat preservation, timed-stop, and precedence logic
• ./gradlew check passes (detekt + all tests + lint)
• API compatibility baseline updated via ./gradlew apiDump

🤖 Generated with Claude Code

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch user-claims

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[github-actions]

Code Coverage

File	Coverage [87.27%]
chat/src/commonMain/kotlin/com/ably/chat/ChatApi.kt	86.34%
chat/src/commonMain/kotlin/com/ably/chat/GsonUtils.kt	93.33%
chat/src/commonMain/kotlin/com/ably/chat/Message.kt	88.89%
chat/src/commonMain/kotlin/com/ably/chat/MessageReactions.kt	84.96%
chat/src/commonMain/kotlin/com/ably/chat/Messages.kt	87.69%
chat/src/commonMain/kotlin/com/ably/chat/Presence.kt	87.27%
chat/src/commonMain/kotlin/com/ably/chat/RoomReaction.kt	100.00%
chat/src/commonMain/kotlin/com/ably/chat/RoomReactions.kt	78.43%
chat/src/commonMain/kotlin/com/ably/chat/Typing.kt	94.57%

Total Project Coverage	85.23%	✅