Previously the env fallback ran only when JSON.parse threw. If stdin was valid
JSON but omitted transcript_path or provided a non-string/empty value, the
script dropped to the getSessionIdShort() fallback path, re-introducing the
collision this PR targets.
Validate the parsed transcript_path and apply the env-var fallback for any
unusable value, not just malformed JSON. Matches coderabbit's outside-diff
suggestion and keeps both input-source paths equivalent.
Refs #1494
0c3fc7074e