fix: improve SVG icon cache validation

[18202781743]

1. Added QFileInfo check for SVG file existence and lastModified
   validity before cache processing
2. Moved svgFileInfo creation outside conditional block for reuse
3. Ensures cache invalidation properly checks file modification times
4. Prevents potential cache issues with invalid file timestamps

fix: 改进SVG图标缓存验证

1. 在缓存处理前添加对SVG文件存在性和最后修改时间的QFileInfo检查
2. 将svgFileInfo创建移到条件块外以便重用
3. 确保缓存失效时正确检查文件修改时间
4. 防止因无效文件时间戳导致的潜在缓存问题

[deepin-ci-robot]

deepin pr auto review

关键摘要:

• 在新添加的代码中，svgFileInfo 对象被重复创建，建议将其移动到条件判断之外以提高效率。
• 注释中提到的问题（ostree管理的源文件 lastModified 被重置为1970-01-01）需要进一步确认是否会影响功能，并考虑是否有其他解决方案。

是否建议立即修改:

• 是，建议立即修改重复创建 svgFileInfo 对象的问题，以提高代码效率。
• 是，建议进一步调查注释中提到的问题，并确保代码的健壮性。

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: 18202781743, mhduiy

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