Return response from streamed request to Anthropic

What does this MR do and why?

Return response from streamed request to Anthropic

This accumulates the response we get from a streamed request to Anthropic and returns it as one string.

This is helpful as we need the full response when broadcasting the messages from chat.

The tests were not correct as the streamed response contained one additional \n.

It also fixes how we parse the SSE as it's not guaranteed that we receive only one SSE from the HTTP.post yield.

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Before	After

How to set up and validate locally

a = Gitlab::Llm::Anthropic::Client.new(User.first).stream(prompt: "What is 1+1?") do |e|
end

puts a

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