fix: handle partial prompt response and add unit tests

[sitaram-kalluri]

With the current logic in messageHandler, if a partial prompt response is received, then the partial responses gets appended to subsequent responses.

- What I did

1. Refactor the messageHandler code as follows:
   • Add data to the buffer until we encounter a terminating character (new line character - \n)
   • Decode the data to the String format.
   • Trim the prompt the from the String format
   • Check if response as partial prompt appended from the previous responses. If yes trim it as well

[gkc]

I think we're good to go, here; we need a solid block of apps testing now and need to be on the lookout for situations we hadn't anticipated