test: implement test helpers and initialization tests (#607)

[BigBen-7]

Description:
This PR establishes the foundational testing boilerplate for the workspace_booking smart contract. It introduces reusable helper functions for the test suite and verifies the core one-time initialization logic of the contract.
closes #607

Key Changes:

• Test Module Setup: Created the contracts/workspace_booking/src/test.rs file with the #![cfg(test)] attribute and necessary Soroban SDK imports.

• Shared Helper Functions: * setup_contract: Registers the WorkspaceBookingContract and returns its address.

• setup_token: Registers a mock Stellar Asset Contract, mints tokens to a recipient, and returns the token address.

• advance_time: A utility to fast-forward the ledger timestamp for time-based testing.

• Initialization Tests:

• test_initialize_success: Verifies that calling initialize correctly sets the admin and payment token using mock_all_auths().

• test_initialize_twice_fails: Ensures that attempting to initialize an already initialized contract panics with the expected AlreadyInitialized = 3 error.

Acceptance Criteria Met:

• cargo test -p workspace_booking test_initialize_success passes.
• cargo test -p workspace_booking test_initialize_twice_fails passes.
• All 3 helper functions (setup_contract, setup_token, advance_time) are present and functional.
• cargo clippy -p workspace_booking -- -D warnings passes with no warnings.

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[yusuftomilola]

Thanks for your contribution