Function apply_pending_migrations 

pub fn apply_pending_migrations<T>(
    account: &AccountView,
    current_epoch: u32,
) -> Result<u32, ProgramError>
where
    T: AccountLayout + LayoutContract + LayoutMigration,

Apply all pending migrations needed to bring the account at current_epoch up to AccountLayout::SCHEMA_EPOCH.

Returns Ok(applied_count) if everything up-migrated cleanly. Returns Err(MigrationMismatch) if the declared chain is incomplete, non-monotonic, or doesn’t start at current_epoch. Returns Err(MigrationRejected) if a user migrator function returned an error.