Problem:
1. The original info text is truncated in pum_set_preview_text.
2. There is an extra newline character at the end of the info text.
3. Incorrect usage of plines_win leads to errors in calculating the window height.
Solution:
1. Improved string handling in pum_preview_set_text to safely process line content without modifying the original text. Now, the preview info is correctly preserved while splitting lines.
2. Do not append a trailing newline to the preview buffer when one is present at the end.
3. Set w_widher_inner in advance; otherwise, plines_win cannot correctly calculate the wrapped line height.
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