Function buildRichAdaptiveCard

Build the complete Adaptive Card for an agent response.

Layout:

┌─────────────────────────────────┐
│ [Avatar] **Agent Name**         │  ← ColumnSet (agent header)
│─────────────────────────────────│  ← separator on next element
│ Response text (TextBlocks)      │  ← from splitMarkdownIntoSections
│                                 │
│ ┌─ Artifact link ─────────────┐ │  ← Container (if artifact/conversation)
│ │ View in MJ Explorer         │ │
│ └─────────────────────────────┘ │
│                                 │
│ Completed in 4.2s · 1,240 tok  │  ← TextBlock (subtle footer)
├─────────────────────────────────┤
│ [Action1] [Action2] [Explorer]  │  ← actions[] array
└─────────────────────────────────┘
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