Fix AI freeze when declaring attackers with unpayable attack costs

[vanja-ivancevic]

• AI would freeze in an infinite loop when declaring attackers that have attack tax costs (e.g. Propaganda) they cannot pay, forcing the player to concede
• The bug required interdependent attackers like two Mogg Flunkies (which can only attack if another creature also attacks): the AI would declare both, PhaseHandler would remove one for not paying the tax, the remaining one alone would fail validation, and the loop would restart with the AI making the same decision
• Reproduced by destroying the AI's lands at the beginning of combat while Propaganda was in play
• Fix: check attack cost payability at declaration time in AiController, before the attackers reach PhaseHandler's payment loop. This aligns with CR 508.1d which says the decision to pay attack costs is made during attacker declaration, not as a separate step afterward

[tool4ever]

nice find