feat: Treat HTTP request completion as final response

[nnshah1]

For HTTP streaming endpoints which do not explicitly send a [DONE] message, updated to treat the HTTP request completion as a final response.

This is backwards compatible and doesn't effect endpoints that do explicitly send [DONE].

This is one change needed for Triton Generate as well as some TGI interfaces / servers.

Checks are added to ensure only a single final response is sent.

[dyastremsky]

Do you have a quick repro/test we could do to the see the effect e.g. with the generate endpoint? We might want to add a CI test to verify this behavior.

[nnshah1]

Do you have a quick repro/test we could do to the see the effect e.g. with the generate endpoint? We might want to add a CI test to verify this behavior.

will require further changes to support generate - was thinking to seperate the C++ changes from the python changes to make merging easier - but we ccould do an feature branch - with seperate MRs into it - if preferred -

I'm good either way