Fix a set of issues around transfer and save of custom editor documents

[dmitrivMS]

Fixes #125293

In addition to original scenario in the bug (error/duplicate opened documents) there were more issues down the line:

• Create untitled doc, enter some text
• Reopen in Hex Editor -> original bug
• Save from Hex Editor -> the untitled document was left around and a "save unsaved changes" dialog was popping up -> fixed
• Now create a new untitled document, enter some text
• Reopen the new doc in Hex Editor -> we were crashing

With these changes all the above issues are fixed and work as expected.

Moving to Jan milestone since it's a larger change.

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@bpasero

Matched files:

• src/vs/workbench/services/untitled/common/untitledTextEditorModel.ts

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