Fixes #4960.

This updates the childLoggerFactory type surfaces to accept FastifyBaseLogger instead of the concrete logger generic inferred from loggerInstance. That avoids making FastifyInstance<..., pino.Logger> fail assignment to the default FastifyInstance type under strict function types.

The added type test covers assigning a Fastify instance created with loggerInstance: pino() to FastifyInstance.

Verification:

• Red before fix: npx tsd --files test/types/fastify.test-d.ts failed on the new Pino logger instance assignment.
• Green after fix: npx tsd --files test/types/fastify.test-d.ts
• npm run test:types
• npm run lint
• git diff --check
• npm test