refactor(client): split ChatMessage.fromJson into typed helpers

[NC1107]

Clears SonarQube S3776 at chat_message.dart:165 (was CC=27) by breaking the system-event sentinel translator into four single-purpose static helpers. Each branch of the orchestrator now reads as a one-line dispatch.

Behind the scenes

• translateSystemSentinel is now a flat dispatcher over the four known event tags; cognitive complexity drops to 4 (≤ 15 budget).
• Extracted _parseUuidUsername to parse the <uuid>:<username> payload that every event tag shares.
• Extracted _formatActiveEvent for "X joined" / "X left" style strings (first-person "You joined").
• Extracted _formatPassiveEvent for "X was removed" / "X was banned" style strings (first-person "You were removed").
• Extracted _subjectFor to centralise the self-vs-peer subject phrase rendering used by both active and passive events.
• Public API unchanged: ChatMessage.fromServerJson, the ChatMessage constructor, and the JSON wire shape are untouched.

Tests: all 86 model + chat_provider_state_test cases pass (test/models/chat_message_test.dart, test/models/chat_message_extended_test.dart, test/providers/chat_provider_state_test.dart). dart format and flutter analyze --fatal-infos clean.