Function identity_account_validate_stored_impl 

pub(crate) async fn identity_account_validate_stored_impl<'o>(
    state: &AppState,
    owner: impl Into<SelfOwner<'o>>,
    account_id: &str,
) -> Result<IdentityAccountValidateResult, String>

Validate one of the agent’s own linked accounts by probing the provider with the stored keychain secret. Ownership is verified (the account must be linked to agent_id) before the secret is read.