fix(contracts): address issues #234, #235, #236, #237

[ScriptedBro]

Description

This pull request resolves four contract issues across contracts/escrow and contracts/marketplace:

1. Escrow create_listing validates maximum expiry window (Fixes #234)

• Enforces an upper-bound expiry duration MAX_EXPIRY_SECS: u64 = 30 * 24 * 60 * 60 (30 days).
• Returns ContractError::InvalidExpiry if expires_at > now + MAX_EXPIRY_SECS.
• Added unit tests verifying upper-bound rejection and boundary acceptance.

2. Marketplace cancel_and_refund depositor verification (Fixes #235)

• Ensures buyer argument matches the actual depositor recorded in listing.buyer.
• Returns ContractError::Unauthorized if mismatched.
• Added unit test verifying rejection of unauthorized buyer address during refund.

3. Admin transfer / ownership handover functionality (Fixes #236)

• Implemented transfer_admin(env, new_admin) on both EscrowContract and MarketplaceContract.
• Requires current admin authentication (admin.require_auth()).
• Emits topics: ["admin", "transfer"] event containing (old_admin, new_admin).
• Added unit tests covering authorized transfer, unauthorized invocation rejection, and confirming the new admin can execute admin operations immediately.

4. Escrow release_payment event topic correction (Fixes #237)

• Defined dedicated topic_released() (symbol_short!("released")) for individual sub-escrow releases.
• Emits topic_released upon each sub-escrow release.
• Only publishes topic_completed when the trade status transitions to TradeStatus::Completed.
• Added unit test asserting event topics across both partial release and completion stages.

Related Issues

• Fixes [contract] - Escrow create_listing does not validate a maximum expiry window #234
• Fixes [contract] - Marketplace cancel_and_refund does not verify the buyer parameter matches the actual depositor #235
• Fixes [contract] - No admin transfer / ownership handover function in either contract #236
• Fixes [contract] - Escrow release_payment emits wrong event topic for partial sub-escrow releases #237

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