Anti-Spam Session Deposit Burn

[Luluameh]

Background
Bots or malicious actors may open thousands of zero-value sessions to pollute state storage. A small non-refundable deposit deters abuse.
Goal
Require a configurable spam_deposit: i128 (e.g., 0.01 XLM) per session, burned to the insurance fund on creation.
Acceptance Criteria

• spam_deposit_amount configurable by FeeManager.
• Deposit transferred to insurance_vault on start_session.
• Does not count toward the session escrow balance.
• Returns Error::InsufficientAntiSpamDeposit on failure.
  Key Files
  contracts/src/lib.rs, contracts/src/treasury.rs