Add AI billing page to FinOps Hub ADX dashboard

📝 Scenario

As a FinOps practitioner, I need to view AI-specific cost breakdowns (Azure AI Services, Azure Machine Learning, Azure AI Search) in the FinOps Hub ADX dashboard in order to monitor and optimize AI spending at the billing level, separate from general cloud costs, with KPIs, daily trends, service breakdown, and top resource identification.

💎 Solution

Add a new "AI billing" page to the FinOps Hub ADX dashboard (finops-hub-dashboard.json) with 4 tiles:

1. AI cost KPIs (multistat) — Total AI Cost, AI % of Cloud spend, Month-over-Month change %, distinct AI resource count
2. AI cost per day (area chart) — Daily EffectiveCost trend for AI services over numberOfDays
3. AI cost by service (bar chart) — Cost breakdown by ServiceName with share percentage and resource count
4. Top 5 AI resources (table) — Highest-cost resources with daily average

Filter criteria using FOCUS schema columns:

ServiceCategory == 'AI and Machine Learning' or ServiceName == 'Azure AI Search'

This covers Azure AI Services, Azure Machine Learning, and Azure AI Search (which is categorized under "Analytics" in FOCUS, not "AI and Machine Learning").

Navigation tile follows existing dashboard pattern (22-column grid, y:0 nav bar). All queries use existing base variables (CostsByDay, CostsByMonth, CostsPlus) so no new data sources are required.

📋 Tasks

### Required tasks
- [ ] Add "AI billing" page definition to `finops-hub-dashboard.json`
- [ ] Add navigation tile for AI billing page (consistent with existing nav pattern)
- [ ] Add markdown description tile with page context
- [ ] Add AI cost KPIs query using `CostsByDay` and `CostsPlus` base variables
- [ ] Add AI daily trend query using `CostsByDay` base variable
- [ ] Add AI cost by service breakdown query using `CostsByMonth` base variable
- [ ] Add Top 5 AI resources query using `CostsByDay` base variable
- [ ] Add corresponding tile visualizations (multistat, area chart, bar chart, table)
- [ ] Validate JSON structure (no duplicate IDs, schema v60 compliance)
- [ ] Test with FinOps Hub ADX cluster data

Start in finops-hub-dashboard.json by reviewing the existing navigation pattern and page definitions. Use the existing CostsByDay, CostsByMonth, and CostsPlus variables with the stated AI service filters, then validate the JSON structure and test the dashboard with FinOps Hub ADX cluster data. Done means the AI billing page has its navigation, description, four visualizations, and no duplicate IDs.