Migrate Prisma adapter from $use() to $extends() for Prisma v7 compatibility

[queso]

Context

The current Prisma adapter (src/server/adapters/prisma.ts) uses prisma.$use() middleware to intercept create/update/delete operations. This API was deprecated in Prisma v4.16.0 and removed in Prisma v7.0.0 (November 2025).

Required Change

Migrate from $use() middleware to Prisma Client Extensions ($extends({ query: { ... } })), which is the officially supported replacement.

The $extends API provides per-model and per-operation hooks via query:

const prisma = new PrismaClient().$extends({
  query: {
    $allModels: {
      async create({ model, args, query }) { ... },
      async update({ model, args, query }) { ... },
      async delete({ model, args, query }) { ... },
    },
  },
})

Impact

• The current adapter works with Prisma v4-v6
• Users on Prisma v7+ cannot use createPrismaAdapter at all
• The adapter interface (SSEAdapter) stays the same — only the internal registration mechanism changes

Acceptance Criteria

• Adapter uses $extends() instead of $use()
• Works with Prisma v7+
• Existing tests pass (update mocks as needed)
• README updated to remove version constraint note