fix(better-auth): import User/Session types from better-auth

[jycouet]

Closes #1099

Description

The better-auth/minimal entry point only exports betterAuth/BetterAuthOptions, not User/Session. Those types now resolve from the main better-auth entry. The runtime betterAuth import stays on /minimal (type-only change, no bundle/dependency impact).

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[changeset-bot]

🦋 Changeset detected

Latest commit: 26fd74a

The changes in this PR will be included in the next version bump.

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