fix(escrow): require delivery before receipt confirmation

[kongwen686]

Summary

• add a Delivered order status and set it when farmers call mark_delivered
• require orders to be delivered before confirm_receipt can complete payment
• update escrow tests so confirmation happens only after delivery and undelivered confirmation returns OrderNotDelivered
• assert the exact error for repeated delivery marking

Closes #233

Tests

• Not run locally (cargo is not installed in this environment).
• Verified the remote diff to confirm all direct receipt confirmations in the escrow tests now pass through mark_delivered first.

Payment / contact

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[kongwen686]

Payment update: Stripe onboarding is not available for me from mainland China. If this contribution is approved for a reward, can payout be sent via PayPal instead? PayPal receiving account: hooqee@126.com. Contact email: chaoqiang.tian@gmail.com. If PayPal is not supported, Stellar/EVM/Alipay details are available on request / in the PR body.