refactor: Use ensure_context decorator in Actor class

Replace manual `_raise_if_not_initialized()` calls with the `ensure_context`
decorator from Crawlee, matching the pattern already used in ChargingManager.

- Add `active` attribute to `_ActorType` (replaces `_is_initialized`)
- Decorate all public methods with `@ensure_context` instead of calling
  `_raise_if_not_initialized()` at the start of each method
- Remove the now-unused `_raise_if_not_initialized` method
- Update all test references from `_is_initialized` to `active`

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: vdusek