test: add task expiration detection coverage

[coderolisa]

Closes #155

Add a reusable expiration-checking function with comprehensive tests to detect tasks that have passed their deadline.

Changes:

• Add expiration.rs module with is_task_expired() helper function
• Refactor submission.rs to use the expiration helper consistently
• Add 7 unit tests for expiration edge cases (past, future, boundary)
• Add 4 integration tests verifying expiration in submission flow

The helper uses the existing Task.deadline field and compares it with the current ledger timestamp. A task is expired when current_time > deadline.

Boundary behavior: Tasks at exactly the deadline timestamp are NOT considered expired (deadline is inclusive). This follows the existing convention where env.ledger().timestamp() > task.deadline triggers TaskExpired error.

All 38 tests pass, including existing regression test

Checklist

General

• Branch is up to date with main
• PR title follows Conventional Commits format (e.g. feat: add reputation system)
• PR description is complete with issue reference (Closes #<issue-number>)
• One logical change per PR (no unrelated changes bundled in)

Testing

• Tests added or updated to cover the changes
• All tests pass (cargo test for contracts, npm run build for frontend)
• No compiler warnings (cargo clippy -- -D warnings)

Documentation

• Public API changes are documented with /// doc comments (Rust) or JSDoc (TypeScript)
• CONTRIBUTING.md, README.md, or other docs updated if needed
• CONTRACT_API.md updated if contract interface changed

Code Quality

• Code follows the project style (cargo fmt / ESLint)
• No any types introduced in TypeScript
• No hardcoded secrets, keys, or environment-specific values

Smart Contract (if applicable)

• require_auth() called on all state-changing operations
• Events emitted for all state changes
• Storage keys don't collide with existing keys
• Deadline and input validation is complete
• No unintended token transfer paths introduced

Frontend (if applicable)

• Components are under 150 lines
• UI is responsive and accessible
• Screenshots or recordings included for visual changes