Better Auth community helper for ThunderID

[brionmario]

Purpose

Adds @thunderid/better-auth, a community provider helper for Better Auth's Generic OAuth plugin. It returns a typed GenericOAuthConfig for a ThunderID issuer, so a Better Auth app can sign users in through ThunderID without any first-class provider support in Better Auth itself.

This follows the approach requested by the Better Auth maintainers after better-auth/better-auth#10237 (a first-class generic-oauth provider export) was declined per their social provider integration policy — they asked for a community provider helper package instead, to be listed in their docs.

Approach

thunderid() only supplies configuration — the callback route, discovery URL derivation, and scopes — for Better Auth's generic-oauth plugin. All OAuth 2.0 / OIDC handling (authorization, token exchange, callback handling) is performed by Better Auth itself, so there is no ThunderID SDK dependency or protocol logic to keep in sync.

The callback route registered with ThunderID is /callback/:providerId, not /oauth2/callback/:providerId — this matches the Generic OAuth plugin's actual core callback route, verified against a running sample app sign-in.

Related Issues

• Integration: Contribute ThunderID as a pre-configured Better Auth provider thunderid#3559

Related PRs

• N/A

Checklist

• Followed the contribution guidelines.
• Manual test round performed and verified.
• Documentation provided. (README with install/usage/options docs and the callback URL to register)
• Tests provided. (Add links if there are any)
  • Unit Tests
  • Integration Tests
• Breaking changes. (Fill if applicable)
  • Breaking changes section filled.
  • breaking change label added.

Security checks

• Followed secure coding standards.
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets.

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