feat(PlayerExecution): downgrade defense posts on capture (#1957)

## Description:

Closes https://github.com/openfrontio/OpenFrontIO/issues/1619.

On capture, defense posts will be downgraded.

On the live version this means defense posts will be destroyed, as
defense posts can only be level 1.

Misc. changes:
- added `decreaserLevel` helper
- cleaned up if/else in tick unit loop for clarity to avoid yet another
nested layer

Continuation of the stale PR,
https://github.com/openfrontio/OpenFrontIO/pull/1622.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

Discord username: `seekerreturns`

tests/core/executions/PlayerExecution.test.ts

@@ -0,0 +1,95 @@
+import { PlayerExecution } from "../../../src/core/execution/PlayerExecution";
+import {
+  Game,
+  Player,
+  PlayerInfo,
+  PlayerType,
+  UnitType,
+} from "../../../src/core/game/Game";
+import { setup } from "../../util/Setup";
+import { executeTicks } from "../../util/utils";
+
+let game: Game;
+let player: Player;
+let otherPlayer: Player;
+
+describe("PlayerExecution", () => {
+  beforeEach(async () => {
+    game = await setup(
+      "big_plains",
+      {
+        infiniteGold: true,
+        instantBuild: true,
+      },
+      [
+        new PlayerInfo("player", PlayerType.Human, "client_id1", "player_id"),
+        new PlayerInfo("other", PlayerType.Human, "client_id2", "other_id"),
+      ],
+    );
+
+    while (game.inSpawnPhase()) {
+      game.executeNextTick();
+    }
+
+    player = game.player("player_id");
+    otherPlayer = game.player("other_id");
+
+    game.addExecution(new PlayerExecution(player));
+    game.addExecution(new PlayerExecution(otherPlayer));
+  });
+
+  test("DefensePost lv. 1 is destroyed when tile owner changes", () => {
+    const tile = game.ref(50, 50);
+    player.conquer(tile);
+    const defensePost = player.buildUnit(UnitType.DefensePost, tile, {});
+
+    game.executeNextTick();
+    expect(game.unitCount(UnitType.DefensePost)).toBe(1);
+    expect(defensePost.level()).toBe(1);
+
+    otherPlayer.conquer(tile);
+    executeTicks(game, 2);
+
+    expect(game.unitCount(UnitType.DefensePost)).toBe(0);
+  });
+
+  test("DefensePost lv. 2+ is downgraded when tile owner changes", () => {
+    const tile = game.ref(50, 50);
+    player.conquer(tile);
+    const defensePost = player.buildUnit(UnitType.DefensePost, tile, {});
+    defensePost.increaseLevel();
+
+    expect(defensePost.level()).toBe(2);
+    expect(game.unitCount(UnitType.DefensePost)).toBe(2); // unitCount sums levels
+    expect(player.units(UnitType.DefensePost)).toHaveLength(1);
+    expect(defensePost.isActive()).toBe(true);
+
+    otherPlayer.conquer(tile);
+    executeTicks(game, 2);
+
+    expect(defensePost.level()).toBe(1);
+    expect(game.unitCount(UnitType.DefensePost)).toBe(1);
+    expect(otherPlayer.units(UnitType.DefensePost)).toHaveLength(1);
+    expect(defensePost.owner()).toBe(otherPlayer);
+    expect(defensePost.isActive()).toBe(true);
+  });
+
+  test("Non-DefensePost structures are transferred (not downgraded) when tile owner changes", () => {
+    const tile = game.ref(50, 50);
+    player.conquer(tile);
+    const city = player.buildUnit(UnitType.City, tile, {});
+
+    expect(game.unitCount(UnitType.City)).toBe(1);
+    expect(city.level()).toBe(1);
+    expect(city.owner()).toBe(player);
+    expect(city.isActive()).toBe(true);
+
+    otherPlayer.conquer(tile);
+    executeTicks(game, 2);
+
+    expect(game.unitCount(UnitType.City)).toBe(1);
+    expect(city.level()).toBe(1);
+    expect(city.owner()).toBe(otherPlayer);
+    expect(city.isActive()).toBe(true);
+  });
+});