refactor(lexicon): align richtext facets with atProto best practices
Align richtext facet lexicon with atProto Lexinomicon style guide:
- Remove $type from required fields (automatically added by SDK for union discrimination)
- Remove handle field from mention type (use persistent DIDs only per best practices)
- Add maxGraphemes constraint to spoiler reason field for proper internationalization
- Update descriptions to match Bluesky documentation patterns
- Update tests to remove handle field references
References: https://github.com/bluesky-social/atproto/discussions/4245
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Co-Authored-By: Claude <noreply@anthropic.com>