stream_integrity: streaming /v1/completions that fails upstream immediately returns 200 with only data: [DONE] #126

Found while working on nan-devops#642. With stream_integrity registered as a callback, a streaming /v1/completions request whose upstream fails before the first chunk (for example a context-window overflow) gets HTTP 200 with only data: [DONE]. The error is swallowed and the client sees an empty success.

Chat completions and /v1/responses are not affected. Reproducible in the real-litellm harness (the case is excluded from the #642 chain test, with a comment).

Expected: the error should surface, the same as on the non-stream path or on streamed chat.

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