Add test coverage for MediaPickerConfigSerializer v13 migration logic

[Copilot]

Addresses feedback on #873 requesting test coverage for the MediaPickerConfigSerializer migration logic that converts v13 configuration formats to v14.

Changes

• Added MediaPickerMigrationTests.cs following existing migration test patterns
  • Filter migration: aliases → GUIDs ("Image,Video" → "cc07b313-...,f6c515bb-...")
  • StartNodeId migration: UDI → GUID ("umb://media/71332aa7..." → "71332aa7-8bea-...")
  • Idempotency verification (already-migrated GUIDs unchanged)
  • Combined migration scenarios
  • Edge cases: null filters, unknown aliases

Uses Moq to stub IMediaTypeService with test media types, consistent with ListMigrationTests and RichTextMigrationTests patterns.

// Test verifies this transformation:
{
  "filter": "Image,Video",
  "startNodeId": "umb://media/71332aa78bea44f19aa600de961b66e8"
}
// becomes:
{
  "filter": "cc07b313-0843-4aa8-bbda-871c8da728c8,f6c515bb-653c-4bdc-821c-987729ebe327",
  "startNodeId": "71332aa7-8bea-44f1-9aa6-00de961b66e8"
}

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