Consolidate input state transitions into a single code path #27536

Description

Input runtime state is written from many places, with no single owner of the transition logic:

• InputEventListener: six @Subscribe handlers (inputCreated, inputUpdated, inputStopped, inputDeleted, inputSetup, leaderChanged), each deciding what to do from its own ad-hoc read of the current state
• InputLauncher.launch(): registers the state and mutates it from its own executor
• InputRegistry.remove() / stop() / setup(): combine map updates with lifecycle calls
• InputStatesResource: persists desired_state and posts the triggering event
• InputSetupService.shutDown()

These run on an AsyncEventBus concurrently with the launcher pool and REST threads, and each operation is a read-modify-write spread across several calls that the caller has to sequence correctly. The outcome depends on timing. There is also no reconciliation, so a node that misses an event stays diverged from the persisted desired_state until it restarts.

This has been a recurring source of bugs:

• Race condition in input event handling leads to NullPointerException #5106 create-then-update races, input stopped while still starting
• Input does not start after ContentPack ReST creation #10640 content pack input fails to start with "port already in use", fixed by a restart
• Input installed from content pack shows inconsistent running state and does not receive messages #25425 overview shows running, detail page shows not running
• Need separate events for input stopped and input deleted #7812 stopped and deleted were indistinguishable to subscribers
• UX: minor ux bugs regarding input labels that indicate how many nodes are running the input #22588 leader-only and stopped inputs reported misleadingly
• Api does not list stopped inputs #4484 stopped inputs handled identically to deleted ones

Goal

One place that owns input state transitions, so the outcome does not depend on event timing or on each caller getting the sequence right, and so a node converges on the desired state even when an event is missed. The registry, the launcher and the event handlers should not each hold a piece of the logic.

Out of scope

The read path. #25358 already moved input state reads to MongoDB.