refactor(core): remove convex surfaces

[onmax]

Summary

• remove convex-specific module/runtime surfaces and exports
• migrate provider resolution to hook-based core structure
• keep core tests and exports aligned with no-convex surface

Tests

• pnpm vitest run test/database-provider.test.ts test/schema-generator.test.ts test/exports.test.ts
• pnpm typecheck

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/nuxt-modules/better-auth/@onmax/nuxt-better-auth@114

commit: a200990