fix: isolate named client scopes and mixed registration collisions

[melroy89]

Follow-up to #201. That PR fixed names inherited from Object.prototype when creating a fresh named registry. Two additional combinations still fail: child registrations mutate an inherited parent registry, and registering __proto__ after an unnamed client changes its prototype and bypasses duplicate detection.

Create a local registry when a child adds a named client, retaining inherited clients while isolating parent and sibling scopes. Define named clients as own data properties, including when an unnamed client already exists. Independent siblings can reuse names; collisions with inherited names and actual client properties still fail.

These fixes were developed in my MariaDB fork in melroy89/fastify-mariadb#4, which also addresses startup cleanup. This PR ports only the registration fixes and their regression coverage to the mysql2-based plugin.

Validation with mysql2 3.24.4, Node.js 24, and isolated MySQL 9.7.2:

• npm run lint and all 15 TypeScript assertions passed.
• 55 runtime tests passed with 100% statement, branch, function, and line coverage.
• The original local registration reproduction script now confirms scope isolation, independent sibling names, and safe duplicate detection after an unnamed registration. The script remains outside the repository.