Improved Transcript/JSONL File Validation

Reading transcript files now provides clearer error messages when files are malformed:

• "No messages found in JSONL file" - when JSONL file parses but contains no messages
• "No valid conversation chain found in JSONL file" - when messages exist but no valid chain can be reconstructed
• "Invalid JSON in transcript file" - when JSON parsing fails
• "Transcript messages must be an array" / "Transcript must be an array of messages or an object with a messages array" - format validation