* op-service: Add ActiveL2EndpointProvider.

* Fix bug in initialization, and handle case where no ethUrls are provided.

* Split active L2 provider into active rollup and active L2 provider.

* Re-duplicate some code until tests are passing.

* op-proposer: Add ability to enable active provider.

* op-batcher: Add ability to enable active provider.

* Add an empty test skeleton.

* Add an empty test skeleton.

* op-service: add, but do not yet use, RollupClientInterface and EthClientInterface.

* op-service: update mocks and interfaces for endpoint provider testing.

* op-service - WIP on Active L2 Providers: unit tests pass, design and impl contains TODOs.

* op-service: restore design in Active Endpoint Providers that only keeps one client open at a time.

* op-service: when dialing a new sequencer, close() the old connection.

* op-service: obey coderabbit suggestion around safer handling of p.currentIndex in Active L2 Providers.

* op-service, op-batcher, op-proposer: address review comments in PR#8585.

* op-service: Active L2 Provider - add test case for a sequencer returning an error.

* op-service: Active L2/Rollup Providers: improve unit testing and logging.

* op-service, op-batcher: address review comments in 8585 regarding first-startup behavior and testing.

* op-service: address review comments through adding more tests, and moving "nil client" behavior from client getter to constructor.

* op-service: minor error message change in active endpoint providers.

* Update op-service/dial/active_l2_provider.go
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

* op-service: obey linter in rabbit-provided error message change.

* Update op-service/dial/active_l2_provider.go
Co-authored-by: Sebastian Stammler <seb@oplabs.co>

* op-service active L2 provider tests: assertAllExpectations after most tests.

* op-service: more elegantly handle startup in active l2 providers, and improve testing.

* Change remaining longDurationTests to be able to use ept.assertAllExpectations.

* use new error errSeqUnset.

* Add test for scenario where many sequencers are inactive, and only the last is active.

* Readability change: move the on-creation initialization to its own function.

* Move extra one-time dial to constructor.

* Update op-service/dial/active_rollup_provider.go
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

* Add nil check to active l2 provider.

* Update op-service/dial/active_rollup_provider.go
Co-authored-by: Sebastian Stammler <seb@oplabs.co>

* Address review comment: change many-inactive tests to many-undialable tests.

* Add test that reproduces internal state corruption.

* op-service: Improve active seq provider

- Preserve the invariant that the index and current rollup/eth
  client match.
- Dial at the start of the loop instead of at the end.

* Fix some tests.

* Move usage of ExpectClose to MaybeClose, we don't want to enforce a particular close behavior in these tests.

* add a missing call to assertAllExpectations.

* Test even the case where the active providers are managing a list of 1 element.

* Revert experimental hunk in active_l2_provider.

---------
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>
Co-authored-by: Sebastian Stammler <seb@oplabs.co>