fix regex in smoothStream()

[carlbrugger]

The word-based regex pattern /\s*\S+\s+/m requires trailing whitespace after each word. The last text-delta won't have a trailing space causing the last word in the last text-delta to not be returned until after any tool calls the model may make. The proposed solution is to match words even if they don't have trailing whitespace.

Resolves #5493

[lgrammel]

I have doubts re the replacement regexp. Won't it match way to much, causing half-finished words to be streamed out?

Instead I'll look into flushing the buffer when there is a tool call, which should cover all cases.

[carlbrugger]

@lgrammel I've changed the implementation to flush the buffer when the chunk type changes.