fix(backend): Handle missing content in streaming delta (#316)

## Summary

This PR fixes a `KeyError: 'content'` that occurs when processing
streaming chat completions.

## Details

When using the `chat_completions` endpoint with `stream=True`, the final
`delta` chunk sent by the server may not contain a `content` key. This
is part of the standard API behavior to signal the end of the stream.

The existing code in `_extract_completions_delta_content` did not
account for this possibility and tried to access `delta['content']`
directly, leading to a `KeyError` and causing the benchmark process to
crash when the stream ended.

## Test Plan

This was discovered while running `guidellm benchmark` against an
OpenAI-compatible API endpoint (via `litellm`) that correctly implements
the streaming protocol.

guidellm benchmark \
  --target "http://10.64.1.62:4000/v1" \
  --model "qwen3-06b-2" \
  --processor "Qwen/Qwen3-0.6B" \
  --rate-type "synchronous" \
  --max-requests 1 \
  --data "prompt_tokens=32,output_tokens=32,samples=1"

## Related Issues

#315

- Resolves #

---

- [x] "I certify that all code in this PR is my own, except as noted
below."

## Use of AI

- [ ] Includes AI-assisted code completion
- [ ] Includes code generated by an AI application
- [ ] Includes AI-generated tests (NOTE: AI written tests should have a
docstring that includes `## WRITTEN BY AI ##`)

Signed-off-by: xinjun.jiang <[email protected]>
Co-authored-by: Samuel Monson <[email protected]>

Author: git-jxj

src/guidellm/backend/openai.py

@@ -688,7 +688,7 @@ def _extract_completions_delta_content(
             return data["choices"][0]["text"]
 
         if type_ == "chat_completions":
-            return data["choices"][0]["delta"]["content"]
+            return data.get("choices", [{}])[0].get("delta", {}).get("content")
 
         raise ValueError(f"Unsupported type: {type_}")