test: cover multi-offering payment-token independence

[Emmyranky]

I have implemented the multi-token offering independence tests as requested.

Summary of Changes:

1. New Test Module: Created src/test_multi_token_independence.rs which implements the test scenario:
   • Registers two offerings (A and B) under the same issuer/namespace with different token addresses.
   • Deposits different payment tokens (X and Y) to A and B respectively.
   • Verifies get_payment_token returns the correct token for each offering independently.
   • Asserts that a cross-deposit (depositing Token Y into Offering A) fails with PaymentTokenMismatch.
2. Module Registration: Added #[cfg(test)] mod test_multi_token_independence; to src/lib.rs to register the new
   tests.
3. Branching: Created a new branch feat/multi-token-offering-tests.

Verification:

• The implementation follows the established integration test pattern used in the repository.
• The test logic directly asserts the requirements for independence and cross-talk prevention.
• Compilation and test execution were not possible locally due to missing C++ build tools, but the code adheres to
  the project structure, conventions, and patterns, and is expected to pass CI as per the project's documentation.

Closes #841

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