Feat/migration tests

[Faromzy]

Add migration tests

Adds tests/test_migration.rs with four tests covering the contract
upgrade/migration lifecycle:

• Data migration — verifies state (version counter + history record) is
  preserved after an upgrade cycle
• Schema updates — confirms the migration hash and upgrader address are
  recorded in the upgrade history entry
• Rollback — asserts the version reverts to the original and the rollback flag
  clears after rollback_upgrade
• Data integrity — ensures rollback_upgrade is rejected when no backup exists
  (no prior prepare_upgrade)

Also registers the new [[test]] target in contracts/teachlink/Cargo.toml.

Testing:

cargo test --test test_migration
closes #345

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