Add AI Observer Panel: per-handler test scaffolding

[droarty]

Summary

• Adds a new defaults action type (server DatabasePersistor.ts via $mergeObjects aggregation update, client documentModelStore.ts, shared ActionItem type) — an object-level upsert-if-absent that adds missing keys without touching existing ones.
• workflow-builder.json's "AI Requests" tab now upserts a state.tests object (one [] array per currently ai-routed handler) whenever opened, then renders the new aiObserverPanel component bound to @state.tests, replacing the previous read-only logTreePanel.
• New aiObserverPanel component (apps/web/src/components/layout/AiObserverPanel.tsx, adapted from LogTreePanel): left side is a tree of handlers → their tests; selecting any node shows an "Add Test" button.
• New get-handler-test-skeleton query: scans a handler's transform/condition/query fields (deliberately excluding its AI prompt text, to avoid false-positive matches) for state.*/message.* references and builds a matching skeleton test object.
• "Add Test" appends the scaffolded skeleton via the existing append action with a dynamically-computed $state.tests.<handlerName> path, and the new test auto-selects once it appears.
• New JsonEditor component shows the selected test's data, freely editable locally — no save/persist wiring yet, per the intended scope of this PR (follow-up work).

Test plan

• Verified live via WebdriverIO against the real running app:
  • The defaults upsert adds [] for new ai-routed handlers without clobbering an existing handler's test array (checked directly against MongoDB)
  • Only ai-routed handlers get tests entries; non-ai handlers do not
  • Clicking a handler selects it and shows "Add Test"
  • "Add Test" scaffolds a skeleton correctly capturing state.*/message.* paths referenced by the handler's transform, while excluding decoy references embedded in its AI system prompt
  • The new test is auto-selected and its skeleton renders correctly in the JSON editor
  • The JSON editor is freely editable locally with no crash
• npx nx build web and npx nx build api both pass
• All temporary test scaffolding (spec files, test users/artifacts/channels/logs) were removed afterward, scoped by exact ID

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