server: do not remove whitespace at the start of a completion chunk (reapply to new UI)

[mgroeber9110]

This merges the change in #7524 to the new UI to keep the two implementations in sync.

Original description:

When using Completion mode of the server in recent builds, the first token of the predicted text is typically appended to the prompt without a space character. This can lead to problems when manually editing the text and then predicting again, because the missing space will appear in the new prompt.

When looking at this change again, I noticed that the code is also used in Chat mode, but in this case HTML rendering removes the whitespace at the start of a conversation turn (after "assistant"), and the conversation transcript looks to be okay afterwards.

[mofosyne]

@mgroeber9110 is the CI issue related to your code or master branch?

[mgroeber9110]

Looking at the error log, this appears to be related to the test environment for Windows:

      Traceback (most recent call last):
        File "C:\hostedtoolcache\windows\Python\3.11.9\x64\Lib\asyncio\windows_events.py", line 494, in finish_recv
          return ov.getresult()
                 ^^^^^^^^^^^^^^
      OSError: [WinError 64] The specified network name is no longer available

This looks unrelated to my change.

[mofosyne]

Okay, then it's most likely due the issue a few weeks ago. Going to merge in directly anyway