Expand test coverage to 43 tests across all layers

[Valamorde]

Summary

Adds 23 new tests (up from 20) to cover every previously untested handler, service action, and repository. Coverage now spans all layers of the codebase.

New test files:

File	What it covers
tests/lambda/keepalive.test.ts	Keepalive Lambda returns 200 immediately
tests/lambda/sendmessage.test.ts	Router: keepalive short-circuit, cloud flag gating, switch_scene forward suppression
tests/handlers/messages/playerjoin.test.ts	Sends fetchscene with stored/null player scene ID
tests/handlers/messages/switch-scene.test.ts	DM and player scene persistence, fetchscene broadcast
tests/handlers/messages/draw.test.ts	Draw data persisted with correct objectId
tests/handlers/services/getScene.test.ts	Assembles scene with tokens, fog, and draw from sub-items
tests/handlers/services/migrate.test.ts	Batch-writes all scene sub-items; generates IDs for scenes missing one
tests/handlers/services/exportScenes.test.ts	Assembles multi-scene export; handles empty campaign
tests/repositories/connection.test.ts	put (DM/player objectId, TTL), deleteById, queryByCampaign, queryByType, findByConnectionId GSI

All tests mock only the DynamoDB and ApiGateway clients — no business logic is stubbed. Tests assert on the specific commands, keys, and payloads that reach AWS, not on internal implementation details.

Test plan

• pnpm test — 43 tests pass across 17 files