This is effectively not possible because it requires a tab stop outside the paragraph indents. When the
Tab for the page number is met, Writer notes that there are no more tabs between the indents and forces a line break to reach the next available tab.
However if you accept to format your TOC in an uncommon way, it is possible to achieve an equivalent to you goal:
I have sent the page number at left instead of the usual right. This presentation requires tweaking both the Contents n paragraph styles and the
Entries Structure in the TOC configuration dialogue.
If it is not clear that the first number is the page number, you can assign it a distinctive character style or a prefix before it. I did both in the following variant:
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