chore(deps): migrate contracts and examples to OpenZeppelin Contracts 5

[alainncls]

Summary

Upgrade the OpenZeppelin contracts stack to the latest eligible major versions older than 3 days as of 2026-04-23:

• @openzeppelin/contracts 4.9.6 -> 5.6.1
• @openzeppelin/contracts-upgradeable 4.9.6 -> 5.6.1

This impacts both contracts and examples.

Why this is not in the current dependency bump PR

This is a breaking contract-library migration. For upgradeable contracts, storage layout and API compatibility must be reviewed carefully before any change is merged.

Documentation

• https://docs.openzeppelin.com/contracts/5.x
• https://docs.openzeppelin.com/contracts/5.x/backwards-compatibility

Expected codebase changes

• Review all imports, inheritance trees, constructors/initializers, and access-control patterns.
• Review removed or renamed helpers, changed hook behaviors, and updated extension APIs.
• Re-check any upgradeable storage layout assumptions before changing live upgrade paths.
• Re-run examples that consume the contracts packages.

Migration plan

1. Inventory every OpenZeppelin base contract and utility used in contracts and examples.
2. Update dependencies to 5.6.1 on a dedicated branch.
3. Resolve compile breaks and explicitly review upgradeable contract storage/layout implications.
4. Re-run all contract tests and add targeted regression tests where inheritance or access control changes.
5. Verify that deployment, upgrade, and example flows still work as expected.

Validation checklist

• Compile all contracts and examples.
• Run the full contracts test suite.
• Review upgrade safety for any upgradeable contracts before considering deployment changes.
• Document any migration constraints discovered during implementation.

[alainncls]

Pas de PR ouverte dans ce passage pour OpenZeppelin 5. Le risque est sur les APIs Solidity et les invariants contrats ; sans revue ciblée des imports/overrides et tests contrat exhaustifs, ce ne serait pas une migration sûre. À traiter séparément de Solhint/Vite.