Add card block support

[VictorHLi404]

Summary

• Adds CardIF for Slack's card block, following the same pattern as vili/table-block-support
• Adds SlackIconObjectIF as a new composition object for the card's slack_icon field
• Reuses existing elements.Image for hero_image/icon and Text for text fields
• actions is modeled as Optional<List<BlockElement>> (flat array, not a wrapped Actions block)
• Registers Card in Block.java's @JsonSubTypes and adds card-specific limits to BlockElementLengthLimits

Test plan

• CardBlockTest covers round-trip deserialization for all 5 payload shapes (slack_icon+actions, hero_image, icon+actions, title-only, actions-only)
• Validation tests assert failures for: missing required fields, icon+slack_icon together, title >150 chars, body >200 chars, actions >3 elements

🤖 Generated with Claude Code

[yyakhnytsia]

Reviewed, left comments.