Implement SessionManager Trait for pallet-session Integration

[motoZ-crypto]

Summary

Replace the placeholder () SessionManager in the runtime with a real implementation provided by pallet-validator, so the active validator set is driven by the validator lock lifecycle (lock / exit / kick / cooldown / lock release) instead of being fixed at genesis.

Changes

• pallet-validator
  • Add pallet-session as a dependency (std + try-runtime features).
  • Add ActiveValidators<T>: BoundedVec<AccountId, MaxValidators> storage
    that mirrors the set currently in use by the session.
  • Implement pallet_session::SessionManager<T::AccountId>:
    • new_session(_) rebuilds the next set as previous ∩ {status == Active}
      plus accounts drained from PendingValidators whose lock status is
      Active. Returns None when the result equals the previous set,
      matching the pallet-session convention. Persists the new set into
      ActiveValidators.
    • start_session / end_session are no-ops in this iteration.

runtime/configs

• Import Validator and set pallet_session::Config::SessionManager = Validator.

Tests

• new_session_promotes_pending_validators
• new_session_removes_exited_validator
• new_session_removes_kicked_and_cooldown_validators
• new_session_returns_none_when_unchanged
• new_session_drops_validator_with_released_lock

Closes #27