Feat/delivery contract initialization

[BigJohn-dev]

Closes #19

Description

This PR implements the unified init function for the Delivery Contract, establishing the essential cross-contract link to the Escrow contract upon instantiation. It refactors the previous fragmented approach by replacing init_admin and set_escrow_contract with a single, atomic initialization path.

Summary of Work Done

• Implemented init(env: Env, admin: Address, escrow_contract: Address) to store both administrative and escrow contract addresses dynamically inside instance storage.
• Enforced an AlreadyInitialized panic to guarantee the contract cannot be re-initialized.
• Successfully seeded the global delivery_counter variable to 0 inside persistent storage.
• Added logic to emit the DeliveryContractInitialized event payload securely on successful resolution.
• Refactored test.rs mock suite: Simplified legacy helper functions by registering the MockEscrow instantly inside setup_test.
• Delivered two new comprehensive unit tests ensuring validation of the event payload (test_init_state_and_event) and double-invocation handling (test_double_init).

Cross-Contract Dependencies Introduced

No new crate dependencies were introduced; however, the logic solidifies the architectural boundary that a Delivery Contract now inherently depends on a valid Escrow Address during the bootstrap phase rather than at runtime lazily.

Test

All unit tests have been strictly verified to pass locally matching the expected acceptance criteria.

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