Prevent AssetsManagerEx destructor from clearing active static instance

[laipg91]

Re: #

Changelog

• Prevent a potential race condition when multiple AssetsManagerEx instances are created sequentially.
• Avoid clearing the active static assetsManager pointer when an older instance is destroyed.
• Ensure native events continue to be dispatched to the current AssetsManagerEx instance during sequential hot updates.

---

Patch

AssetsManagerEx::~AssetsManagerEx()
{
-    assetsManager = nullptr;
+    if (assetsManager == this) {
+        assetsManager = nullptr;
+    }
}

---

Continuous Integration

This pull request:

• needs automatic test cases check.

  Manual trigger with @cocos-robot run test cases afterward.

• does not change any runtime related code or build configuration

  If any reviewer thinks the CI checks are needed, please uncheck this option, then close and reopen the issue.

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Compatibility Check

This pull request:

• changes public API, and have ensured backward compatibility with deprecated features.
• affects platform compatibility, e.g. system version, browser version, platform sdk version, platform toolchain, language version, hardware compatibility etc.
• affects file structure of the build package or build configuration which requires user project upgrade.
• introduces breaking changes, please list all changes, affected features and the scope of violation.

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[github-actions]

Code Size Check Report

Wechat (WASM)	Before	After	Diff
2D Empty (legacy pipeline)	1009838 bytes	1009838 bytes	✅ 0 bytes
2D All (legacy pipeline)	2675490 bytes	2675490 bytes	✅ 0 bytes
2D All (new pipeline)	2767226 bytes	2767226 bytes	✅ 0 bytes
(2D + 3D) All	10024224 bytes	10024224 bytes	✅ 0 bytes

Web (WASM + ASMJS)	Before	After	Diff
(2D + 3D) All	16843286 bytes	16843286 bytes	✅ 0 bytes

Interface Check Report

This pull request does not change any public interfaces !