fix(types): preserve server auth inference for plugins

[onmax]

Summary

• fix defineServerAuth typing with overloads so plugin tuples stay inferred without constraining plugin internals
• keep nuxt-better-auth-infer typing inline in src/module/type-templates.ts and infer user/session fields from plugin schema + additionalFields
• add a regression fixture + test for Route Rules TS not working properly #107 (admin-like plugin schema + user.additionalFields + route rules matcher)
• remove manual playground AuthUser augmentation (/playground/app/types/auth.d.ts)

Tests

• pnpm test test/type-infer-107.test.ts

Notes

• closes Route Rules TS not working properly #107
• follow-up tracking issues: test suite has unrelated failures: custom provider hook + module fixture libsql binary #109 and playground typecheck fails in app/auth.config.ts due to BetterAuthClientPlugin version mismatch #110 (unrelated/pre-existing failures)

[pkg-pr-new]

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npm i https://pkg.pr.new/nuxt-modules/better-auth/@onmax/nuxt-better-auth@111

commit: d760ff9