feat(invoice-escrow): implement batch get_escrows read method

[zinodict121]

Implement batch get_escrows read method

Fixes #87

Description

This PR implements a paginated batch read method for retrieving escrows, addressing the missing iteration capabilities in the contract as requested in issue #87.

Changes:

• types.rs: Added StorageKey::EscrowCount and StorageKey::EscrowIdByIndex(u32) to maintain a global index of created escrows since Soroban cannot iterate persistent map keys natively.
• storage.rs: Added helper functions to manage and read the escrow index.
• lib.rs:
  • Updated create_escrow to append the newly generated invoice_id into the global index.
  • Implemented get_escrows(env, start, limit) function to fetch paginated escrow records.
  • Validates boundaries: rejects limit == 0 (InvalidLimit) and rejects limits above MAX_PAGE_SIZE of 100 (LimitExceeded). Handles start >= total_count gracefully by returning an empty vector.
• errors.rs: Added Error::InvalidLimit (17) and Error::LimitExceeded (18) following the existing enumeration format.
• test.rs: Added comprehensive test cases including empty state, populated page slicing, invalid limits, and pagination boundaries.

Notes

• No events were added since read methods in this contract (e.g. get_escrow) do not emit events.
• Tested boundary parameters meticulously to ensure no integer overflows occur.

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[chizzy192]

Hi @zinodict121, please rebase your branch on dev to resolve line conflicts in contracts/invoice-escrow/src/lib.rs for the batch get_escrows getter implementation.