test(transcription): cover json_format unwrapping
These tests show that the converter reads both payload forms correctly.
The first test sends JSON TimedString chunks. The converter takes only the text field. The times
in the segment stay zero.
The second test sends two chunks that look like JSON but are not. One chunk has no closing brace.
The other chunk parses but has no text field. The converter keeps both chunks without a change,
and it keeps the space at the start of the second chunk. If the converter used the trimmed text,
real transcripts would lose spaces.
