fix: return RuleViolation for invalid trades to prevent AI turn stalls

[njbrake]

Summary

• After Monopoly depletes resources, the AI would repeatedly propose trades offering resources it doesn't have (e.g., offering Wood with 0 Wood). Each invalid trade silently returned Ok(()), consuming action/trade slots and causing the AI turn to last ~2 minutes while the game appeared frozen.
• Changed handle_trade to return Err(RuleViolation) when validate_trade fails, so invalid trades don't count toward limits. Added narration message explaining the failure.
• Added regression test verifying trade validation rejects offers with insufficient resources.

Test plan

• All 431 tests pass (1 new regression test)
• cargo clippy clean
• cargo fmt clean
• Manual test: play a game, have AI play Monopoly card, verify AI turn completes quickly without getting stuck proposing invalid trades

🤖 Generated with Claude Code