Fix message format and refactor types in SSEModel

[AilurusRP]

1. Fixed the issue where received messages contained extra spaces at the beginning of each line and extra newline characters at the end of each message block (A possible solution for Issue#10) .
2. Refactored the types of several fields in the SSEModel class from String? to String to avoid unnecessary null safety checks and reduce code redundancy.

These changes were tested locally and have been verified to work properly. Thank you for considering this pull request!

[pratikbaid3]

@AilurusRedPanda thanks for the fix. Can you change the destination branch to uat. There are a couple of changes there and certain improvements as well. Thanks.

[AilurusRP]

@pratikbaid3 Thanks for your response. I have already updated the target branch to 'uat'.