Rename feature flag workplan_decision_log to decision_log

What does this MR do and why?

Renames the feature flag workplan_decision_log to decision_log across the entire codebase (backend and frontend).

The Decision Log is an independent widget with its own backend logic and APIs that are not coupled to the Work Plan or the aiPlanningEnabled flag. The old name workplan_decision_log implied a dependency that does not exist, causing confusion for contributors. Renaming to decision_log accurately reflects the feature's standalone nature.

Changes:

• Renamed config/feature_flags/wip/workplan_decision_log.yml → config/feature_flags/wip/decision_log.yml (updated name: and description: fields)
• Updated push_frontend_feature_flag calls in ee/app/controllers/ee/groups/work_items_controller.rb and ee/app/controllers/ee/projects/work_items_controller.rb
• Updated glFeatures.workplanDecisionLog → glFeatures.decisionLog in work_item_ai_widget.vue
• Updated comment reference in stub_resolvers.js
• Updated spec file work_item_ai_widget_spec.js to use the new flag name

References

• Issue: #627707 (closed)
• Original discussion: !250010 (comment 3749550850)

Screenshots or screen recordings

N/A — no UI changes, purely a feature flag rename refactor.

How to set up and validate locally

1. Enable the renamed flag in rails console:

   Feature.enable(:decision_log)

2. Visit a work item page and verify the Decision Log widget appears.
3. Confirm the old flag name workplan_decision_log no longer exists.

MR acceptance checklist

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