fix(schema): prefer mjs in hub schema hook

[onmax]

Summary

• prefer schema.<dialect>.mjs over schema.<dialect>.ts in hub:db:schema:extend
• extract schema path selection into resolveHubSchemaPath for deterministic hook behavior
• add focused regression test for path priority and fallback behavior
• document the .mjs priority in NuxtHub integration docs

Why

This fixes builds where Nuxt moves buildDir under node_modules/.cache/nuxt/.nuxt and Node refuses type-stripping for .ts files under node_modules.

Fixes #129

Testing

• pnpm vitest run test/schema-hook-path-priority.test.ts ✅
• pnpm test ⚠️ fails in test/module.test.ts due existing integration environment issues (port/timeouts and transient module resolution under test output) unrelated to this change

[pkg-pr-new]

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npm i https://pkg.pr.new/nuxt-modules/better-auth/@onmax/nuxt-better-auth@131

commit: afd6038