fix(auth): use provider-specific lookup in link-social callbak

[SandipGyawali]

• replaced findAccount() with findAccountByProviderId() in Oauth
• prevents two providers return the same external accountiD

Issue: #8906

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Summary by cubic

Switch the OAuth link-social callback in better-auth to use provider-scoped account lookup to prevent ID collisions across providers. Replace findAccount(...) with findAccountByProviderId(id, provider.id) to ensure the correct user is linked; addresses #8906.

^{Written for commit fbcfc9c. Summary will update on new commits.}

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Project	Deployment	Actions	Updated (UTC)
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commit: fbcfc9c

[cubic-dev-ai]

No issues found across 1 file

[bytaesu]

Could you add regression test code?

[gustavovalverde]

Solved in #8983

[better-release]

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