fix: fix runtime errors with better-auth 1.4.x API changes

[Hashibutogarasu]

Summary

This PR fixes runtime errors that occur when using the plugin with better-auth 1.4.x.

Changes

Fix createAuthMiddleware import path

createAuthMiddleware was moved from better-auth/plugins to better-auth/api in better-auth 1.4.x. Using the old path causes a runtime module resolution error.

Update internalAdapter API calls to match 1.4.x signatures

Several internalAdapter method signatures changed in better-auth 1.4.x:

Operation	Before	After
Update user	updateUser({ id, ...fields })	updateUser(id, { ...fields })
Create session	createSession({ userId, expiresAt })	createSession(userId, undefined, { expiresAt })
Find OAuth user	Manual adapter.getAccount()	internalAdapter.findOAuthUser()
Link account	adapter.upsertAccount / createAccount	internalAdapter.linkAccount() / internalAdapter.updateAccount()

Use GenericEndpointContext instead of hand-crafted type

Replaced the local Context type (which used any) with the official GenericEndpointContext type exported from better-auth, improving type safety.

Fix optional chaining on context.path

In better-auth 1.4.18, context.path can be undefined. Added optional chaining (context.path?.startsWith()) with a fallback to prevent runtime crashes in hook matchers.

Testing

• Verified the plugin works correctly with better-auth 1.4.x
• All existing tests pass