feat(framework): server testing utils, transaction storage simplify, pathfinding tests (#384)

## Server Testing Utils
- Add TestServer, TestClient, MockRoom for unit testing
- Export testing utilities from @esengine/server/testing

## Transaction Storage (BREAKING)
- Simplify RedisStorage/MongoStorage to factory pattern only
- Remove DI client injection option
- Add lazy connection and Symbol.asyncDispose support
- Add 161 unit tests with full coverage

## Pathfinding Tests
- Add 150 unit tests covering all components
- BinaryHeap, Heuristics, AStarPathfinder, GridMap, NavMesh, PathSmoother

## Docs
- Update storage.md for new factory pattern API

packages/framework/server/src/room/RoomManager.ts

@@ -197,9 +197,9 @@ export class RoomManager {
         )
     }
 
-    private _findAvailableRoom(name: string): Room | undefined {
+    private _findAvailableRoom(name: string): Room | null {
         const def = this._definitions.get(name)
-        if (!def) return undefined
+        if (!def) return null
 
         for (const room of this._rooms.values()) {
             if (
@@ -212,7 +212,7 @@ export class RoomManager {
             }
         }
 
-        return undefined
+        return null
     }
 
     private _generateRoomId(): string {