test: add adversarial coverage for set_rounding_mode

[waledev1]

Overview

This PR adds focused adversarial test coverage for the public set_rounding_mode entry point in src/lib.rs. The new test module exercises valid calls, boundary/invalid RoundingMode values, unauthorized callers, and verifies that state is unchanged after rejected operations.

Related Issue

Changes

🧪 Adversarial coverage for set_rounding_mode

• [ADD] src/test_rounding_mode.rs
  • Focused test module targeting set_rounding_mode(env: Env, issuer: Address, namespace: Symbol, token: Address, mode: RoundingMode).
  • Success paths: valid calls with each RoundingMode variant, asserting the stored mode is observable and deterministic.
  • Boundary / invalid inputs: invalid RoundingMode values and boundary namespace/token inputs, asserting rejected calls surface deterministic errors.
  • Authorization: unauthorized issuer callers are rejected.
  • State integrity: after every rejected operation, the previously stored rounding mode is re-read and asserted unchanged.

Verification Results

cargo test test_rounding_mode
✅ focused test module passes

cargo test
✅ surrounding suite passes

cargo clippy --all-targets -- -D warnings
✅ no warnings

cargo build
✅ builds cleanly

Acceptance Criteria	Status
Cover the named behavior with focused automated tests, including the relevant success and failure paths	✅ Valid, boundary/invalid, and unauthorized cases covered
Preserve the existing public contract unless the change includes an explicit compatibility plan	✅ No changes to set_rounding_mode signature or behavior
Make error and boundary behavior observable and deterministic	✅ Rejected calls assert deterministic errors and unchanged state

Closes #1121