Fix Unity 6.5 asset creation API compatibility

[Hinen]

Summary

• Use ProjectWindowUtil.CreateAssetWithTextContent on Unity 6000.5 and newer.
• Keep CreateAssetWithContent on older supported Unity versions.

Why

Unity 6000.5 marks CreateAssetWithContent as an obsolete compile error. The replacement API is not present in Unity 6000.3.11f1, so the version guard must start at 6000.5 rather than 6000.0. This preserves the package's older Unity compatibility.

This differs from #309 by retaining compatibility with Unity 6000.0 through 6000.4.

Verification

• Unity 6000.5.10f1: full project script recompilation completed with failed: false.
• Unity 6000.3.11f1 editor assembly: ProjectWindowUtil exposes CreateAssetWithContent, but not CreateAssetWithTextContent.

[Hinen]

Closing in favor of #309. I added the Unity 6000.3 compatibility finding there so the existing change can be corrected without maintaining a duplicate PR.