fix: Fixed an issue with editor document synchronization

[Kakueeen]

If newly opened files are already open in other tabwidgets, their content changes are not synchronized

Log: fix issue

[deepin-ci-robot]

deepin pr auto review

关键摘要:

• 在handleOpenFile函数中，使用findEditor方法查找打开的文件可能导致性能问题，特别是当文件数量较多时。
• handleOpenFile函数中，对editor的赋值和初始化应该放在循环外部，以避免每次循环都创建新的TextEditor对象。
• handleOpenFile函数中，应该检查document是否为nullptr，以避免在文档为空时调用setDocument方法。
• handleOpenFile函数中，应该检查fileName是否已经存在于tabWidgetList中，以避免重复打开文件。

是否建议立即修改:
是

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Kakueeen, LiHua000
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