fix: update AuthInstance type to use AuthOptions for better type inference

[amaury-tobias]

🔗 Linked issue

Closes #192

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

• Updates server AuthInstance typing to infer from AuthOptions so plugin API types are preserved.
• Adds regression test coverage to verify serverAuth(event) exposes plugin API methods (including signInUsername) with correct typing.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/nuxt-modules/better-auth/@onmax/nuxt-better-auth@193

commit: d38b40a

[onmax]

Thank you!