Add WordRepository integration tests with embedded MongoDB

[Copilot]

• Add EphemeralMongo7 runtime packages
• Create MongoDbTestRunner.cs
• Create WordRepositoryTests.cs (48 integration tests)
• Add [Category("IntegrationTest")] and test:integration / test:unit scripts
• Update WordRepositoryTests.cs to match updated WordRepository:
  • Renamed RepoCreate → Create
  • Renamed RepoUpdateFrontier → UpdateFrontier
  • Renamed RepoReplaceFrontier → ReplaceFrontier
  • Renamed RepoRevertReplaceFrontier → RevertReplaceFrontier (returns bool)
  • Renamed RepoDeleteFrontier → DeleteFrontier
  • Renamed RepoRestoreFrontier → RestoreFrontier (single string arg, returns bool)
  • Reordered test sections to match WordRepository method order
  • Fixed assertions for changed return types

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This change is