fix: fix OSTL ucel merged incorrectly (#3453)

The `_parse_otsl_output` function used to translate VLM OSTL output
into structured table data incorrectly merges ucel to the left
if they are not in the first column. This causes column spanning
cells to be incorrectly reported as row spanning.

Signed-off-by: Sunny He <sunny_he@apple.com>