fix(oauth2): compare accountId as forced string to prevent duplicates

[cemreinanc]

in oauth2/link-account.ts file: accountId returns as a string from database but other one returns as a number from the endpoint. So they dont align and cause hasBeenLinked to be undefined.
in line 46 comparing like this a.accountId === String(account.accountId) seems like solved the issue.

Closes #3814

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Summary by cubic

Fixed an issue where OAuth account linking could create duplicate accounts by ensuring accountId is always compared as a string.

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commit: 6143364

[cubic-dev-ai]

cubic analysis

No issues found across 1 file. Review in cubic

[himself65]

What is the real case causing the accountId to be a number? Could there be a deeper issue behind this? since the type annotation around is all string

[himself65]

I realized that issue is from https://github.com/better-auth/better-auth/pull/3816/files#diff-16e9f39c92f582849ffc6ba9be9b14f8f04a8137bf025e3c876159c01d254564R307 if getUserInfo returns the number. we don't give any error about this

[Bekacru]

closed in favor of #3816