Feat/get escrow state query

[BigJohn-dev]

Closes #13

Description

This PR implements the get_escrow public read-only query function for the Escrow Contract, enabling both frontend and backend layers of the SwiftChain logistics platform to inspect the current state and structure of any escrow record robustly.

Summary of Work Done

• Safely replaced the ambiguous internal get_escrow_record function with the standardized public get_escrow(env: Env, delivery_id: u64) API.
• Introduced the DeliveryNotFound enum natively into EscrowError.
• Implemented state checking upon query execution: the contract now evaluates env.storage().persistent().has(..) accurately before fetching—if the delivery ID does not exist, the mechanism immediately bubbles up an EscrowError::DeliveryNotFound target panic.
• Added Escrow structure dependencies natively to DataKey matching the target lookup specifications.
• Extensively reorganized the escrow_contract/test.rs integration suite to safely utilize the new structured query function everywhere over legacy methods.
• Executed strict edge-case verification via the new test_get_escrow_not_found unit test confirming missing query states evaluate dynamically toward the intended exception behaviors.

Cross-Contract Dependencies Introduced

No new cross-contract bounds were introduced. Operation purely expands persistent storage viewing capacities transparently for the SwiftChain Platform layers.

Test

All unit tests executed transparently across the Soroban SDK. No state mutations occur during reads.

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