test: add adversarial coverage for commit_snapshot in src/lib.rs

[blesswill88]

Overview

This PR adds focused adversarial test coverage for the public commit_snapshot method exposed in src/lib.rs. The new tests exercise valid calls, boundary values for each parameter (env: Env, issuer: Address, namespace: Symbol, token: Address, snapshot_ref: u64, content_hash: BytesN<32>), unauthorized callers, and verify that state is unchanged after rejected operations.

Related Issue

Changes

🧪 Snapshot Finalization Tests

• [ADD] src/test_snapshot_finalization.rs
  • Focused test module covering commit_snapshot success and failure paths.
  • Valid-call coverage: commits a snapshot with well-formed issuer, namespace, token, snapshot_ref, and content_hash, then asserts the resulting state is observable.
  • Boundary coverage for snapshot_ref: u64 (e.g. zero and large values) and content_hash: BytesN<32> (all-zero and all-0xFF hashes).
  • Unauthorized-caller coverage: invokes commit_snapshot from an address that is not the expected issuer and asserts the call is rejected.
  • State-invariance checks: re-reads snapshot state after each rejected operation to confirm nothing was mutated.
  • Deterministic error assertions so failure modes are observable and stable.

Verification Results

cargo test test_snapshot_finalization
✅ focused commit_snapshot tests passed

cargo test
✅ surrounding suite passed

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

cargo build
✅ build succeeded

Acceptance Criteria	Status
Cover the named behavior with focused automated tests, including the relevant success and failure paths	✅ Valid, boundary, and unauthorized paths covered in src/test_snapshot_finalization.rs
Preserve the existing public contract unless the change includes an explicit compatibility plan	✅ No changes to commit_snapshot signature or behavior; tests only
Make error and boundary behavior observable and deterministic	✅ Explicit error assertions and boundary-value cases for snapshot_ref and content_hash

Closes #1137