Refactor message formatting logic into a dedicated, testable class:
- Create MessageFormattingEngine with Patterns enum (precompiled regexes)
- Create MessageFormattingContext protocol for dependency injection
- Extract extractMentions(), containsCashuToken(), and pattern matchers
- Add 28 comprehensive tests for formatting utilities
- Update ChatViewModel to conform to MessageFormattingContext
This prepares for further message formatting extraction and makes
regex patterns and utility functions testable in isolation.
