fix(provider): omit Kakao email scope when optional

[pjpjq]

What changed

• Keep Kakao's existing default behavior of requesting account_email.
• Omit account_email from the default Kakao scopes when the provider is configured with email_optional.
• Preserve an explicit opt-in path by allowing custom scopes to add account_email back.
• Add provider-level scope tests and external Kakao callback tests for the no-email flow.

Why

Supabase's Kakao docs explain that developers can omit account_email and enable "Allow users without an email". The provider currently always requests account_email, which blocks non-Biz Kakao apps before Supabase can handle the no-email user flow.

Fixes #2574.

Validation

• go test ./internal/api/provider
• go test ./internal/api -run 'TestExternal/TestSignupExternalKakao'
• git diff --check