Add tests for DesktopAssetManager cache and asset events

[Dingyi189-eng]

Adds unit tests for DesktopAssetManager caching and AssetEventListener callbacks.

• Verifies cache hits do not fire assetLoaded twice
• Verifies clearCache() forces a reload
• Verifies loadAsset(null) fails fast

Run: ./gradlew :jme3-core:test --tests "com.jme3.asset.DesktopAssetManagerCacheTest"

[gemini-code-assist]

Code Review

This pull request introduces a new test class, DesktopAssetManagerCacheTest, to verify caching and AssetEventListener behavior in DesktopAssetManager. A critical issue was identified in the cachedLoadFiresRequestedButNotSecondLoadedEvent test, where Assert.assertNotSame(first, second) is used incorrectly. Since Texture is a shared asset and not cloned upon cache retrieval, both instances should be identical, and the assertion should be updated to Assert.assertSame to prevent the test from failing.

[gemini-code-assist]

Since Texture is not a CloneableSmartAsset, it is a shared asset and is not cloned upon cache retrieval. Therefore, the first and second loaded textures should be the exact same instance. Using Assert.assertNotSame(first, second) will cause the test to fail. It should be changed to Assert.assertSame(first, second).

Suggested change

	Assert.assertNotSame(first, second);
	Assert.assertSame(first, second);

References

1. Issues found in test code should be reported with a reduced priority, at most medium.