test: add adversarial coverage for get_revenue_by_period

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Overview

This PR adds focused adversarial test coverage for the public get_revenue_by_period contract in src/lib.rs. The new test module exercises valid calls, boundary values, unauthorized callers, and verifies that state remains unchanged after rejected operations.

Related Issue

Changes

🧪 Revenue-by-Period Test Coverage

• [ADD] src/test_revenue_by_period.rs
  • Focused test module covering get_revenue_by_period(env, issuer, namespace, token, period_id).
  • Success paths: valid Env, Address issuer, Symbol namespace, Address token, and representative period_id values.
  • Boundary values: period_id = 0, u64::MAX, and periods with no recorded revenue.
  • Authorization paths: unauthorized caller rejection and confirmation that rejected calls leave storage unchanged.
  • Deterministic assertions on returned values and error behavior.

Verification Results

cargo test --test test_revenue_by_period
✅ focused suite passed

cargo test
✅ surrounding suite passed

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

cargo build
✅ build succeeded

Acceptance Criteria	Status
Cover named behavior with focused automated tests, including success and failure paths	✅ Valid, boundary, and unauthorized cases covered in src/test_revenue_by_period.rs
Preserve existing public contract	✅ No changes to get_revenue_by_period signature or behavior
Make error and boundary behavior observable and deterministic	✅ Explicit assertions on boundary period_id values and rejected unauthorized calls
State unchanged after rejected operations	✅ Post-rejection state assertions included

Closes #1086