Issue #3586657: Allow Field Widget Actions to target a specific automator 2.x

Description

Changes:

• AiAutomatorEntityModifier::saveEntity() gains an optional $specificAutomatorId parameter. When set, processing is filtered to the matching automator config entity ID before any processor runs.
• Unlike 1.x, 2.x's entityHasAutomators() already returns automators keyed by config entity ID and sorts them with a key-preserving uasort(), so the filter here operates directly on that keyed array instead of 1.x's array_filter() placed before a key-destroying usort().
• Added testSaveEntityFiltersBySpecificAutomatorId() and testSaveEntityReturnsNullForUnmatchedAutomatorId() to TextToImageMediaFieldWidgetProcessingTest.

Out of scope for this MR — needs reviewer input:

1.x's fix also touched AutomatorFieldWidgetActionTrait::populateAutomatorValues() (passing $automator_id through to saveEntity(), plus a NULL guard) and AutomatorRefinableBaseAction::generateContent() (same fix, for the "refine" feature). Neither file exists on 2.x yet — both are introduced by the not-yet-merged !1966 (merged) (#3586655 (closed)), which will have the exact same "runs every automator on the field" bug in its generateContent() path once it merges, since it's built on the pre-fix saveEntity() signature.

Current caller of saveEntity() on 2.x (AutomatorBaseAction::populateAutomatorValues()) still calls it without the new parameter, so this MR alone does not yet close the user-facing bug for the default Field Widget Actions button path on 2.x — it only adds the capability the caller needs.

Needs a maintainer decision on sequencing:

• (a) add the $automator_id/NULL-guard fix as a follow-up commit directly on !1966 (merged)'s branch, so it merges together with (or as part of) that MR, or
• (b) merge this MR first, then add that fix here (or in a fast-follow) once !1966 (merged) lands on 2.x and the trait/AutomatorRefinableBaseAction exist to update.

Either way, AutomatorBaseAction::populateAutomatorValues() needs the same two changes once its target file stabilizes post-!1966.

Testing instructions

Checklist

• I have linked the related issue in the MR title or description
• I have performed a self-review of my own code
• I have added or updated tests, or explained in the description why this change is not covered by tests
• I have updated documentation for any new or changed functionality
• I have written testing instructions and verified them locally
• I have noted any required post-merge steps (config imports, cache rebuilds, manual changes)
• This MR contains no breaking API or hook changes, or they are explicitly documented in the description

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Closes #3586657