Temporary storage for session verification

[anonfedora]

Description

This PR implements a temporary session verification system for students completing quizzes, as specified in issue #135. The system leverages Soroban's Temporary Storage to manage short-lived verification codes securely and efficiently.

Key Changes

• New Session Module: Created session.rs which contains the SessionVerificationContract.
• Session Generation: Implemented start_session to generate a unique 128-bit verification code (via SHA-256 hash truncation) and store it with a configurable TTL (100–200 ledgers).
• Verification Logic: Added verify_session to validate student codes against temporary storage.
• Session Extension: Added extend_session to allow active students to prolong their session's validity.
• Module Registration: Registered the session module in the main contract lib.rs.
• Linter Cleanup: Resolved compiler diagnostics and linter warnings in token.rs (unused imports and redundant assertions).

Reference Issues

Fixes #135

Type of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue - Cleanup of compiler diagnostics)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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Test Results

Unit tests for the new session system were added to tests.rs and passed successfully:

running 3 tests
test tests::test_session_expiration ... ok
test tests::test_session_start_and_verify ... ok
test tests::test_session_extension ... ok

Existing token tests also pass after the linter fixes:

running 21 tests
...
test result: ok. 21 passed; 0 failed; 0 ignored; 0 measured; 66 filtered out; finished in 0.18s

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[ayomideadeniran]

Pr under review.