connectors: fix elasticsearch_source cursor overrides runtime state on open

Split out of the #3795 review at hubcio's suggestion.

With `[state] enabled = true`, `elasticsearch_source` persists its own cursor at close and applies it at open, overriding the state the runtime restored. A runtime-side guarantee therefore cannot cover this source: whatever position the runtime hands back, the plugin's own file wins.

That matters for the guarantee #3795 documents. After a dropped batch the runtime's contract is "resume from the last delivered batch's state", and this source can resume from somewhere else entirely.

It is opt-in and off by default, so it is not a live regression, but the two mechanisms should not both own the cursor. Either the plugin defers to the runtime state when one is present, or the plugin-side option is documented as replacing the runtime's guarantee rather than supplementing it.

Happy to take it once the direction is settled.

Start by tracing the elasticsearch_source open and close cursor handling alongside the runtime state restoration described in #3795. Confirm which state is applied after a dropped batch and settle whether runtime state or the plugin cursor owns recovery when both are present. Done means the ownership rule is decided and covered by a regression test.

Written by the indexing model from the issue text.