Add migration tests for ContentPicker and MediaPicker configuration serializers

[Copilot]

The ContentPickerConfigSerializer and MediaPickerConfigSerializer migration logic lacked test coverage, despite similar serializers having corresponding tests in uSync.Tests/Migrations.

Changes

• ContentPickerMigrationTests.cs: 8 tests verifying filter/startNodeId migration

  • Alias → GUID conversion for content type filters
  • UDI → GUID conversion for startNodeId
  • Idempotency (pre-migrated values unchanged)
  • Multi-value and mixed scenarios

• MediaPickerMigrationTests.cs: Parallel test suite for media picker using IMediaTypeService

Both follow existing patterns (MigrationTestBase, Moq for service mocking) and verify the GetConfigurationImport method correctly transforms v13 configuration data:

// Before migration (v13 format)
{
  "filter": "myContentType,anotherType",
  "startNodeId": "umb://document/a1b2c3d4e5f67890abcdef1234567890"
}

// After migration
{
  "filter": "guid1,guid2",
  "startNodeId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890"
}

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