feat(nuxt-better-auth): optional app session enrich hook

[onmax]

Summary

• add optional appSession.enrich to defineServerAuth
• wire enrichment through Better Auth custom-session plugin in module runtime
• merge enrichment as partial user / session updates while preserving default shape
• guard against conflicting manual custom-session plugin configuration
• document usage and types
• add tests for plugin injection, default behavior, and conflict handling

Closes #138

[pkg-pr-new]

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npm i https://pkg.pr.new/nuxt-modules/better-auth/@onmax/nuxt-better-auth@141

commit: 2997752