[DOCS] Chat example says role-only stream chunks validate, but the model rejects them #147

Summary

The Streaming Chat Completion snippet in examples/chat/README.md says "The first chunk carries only role (no content) and validates fine." The code contradicts this. The generated ChatCompletionStreamResponseDelta declares content: str with no default (inference_gateway/models.py:2408), and ChatCompletionStreamChoice.finish_reason is a required FinishReason whose literal does not allow null (inference_gateway/models.py:2479 and :800-803). The repo's own strict xfail test test_stream_response_delta_allows_missing_content (tests/test_client.py:487-511) asserts that ChatCompletionStreamResponseDelta.model_validate({"role": "assistant"}) raises ValidationError. The snippet only catches json.JSONDecodeError, so a role-only chunk, or any chunk with "finish_reason": null, raises an uncaught pydantic.ValidationError out of the loop. examples/chat/main.py, which the README tells users to run, uses the same except json.JSONDecodeError-only pattern. The root README.md "Streaming Content" section already handles this correctly: it catches (json.JSONDecodeError, ValidationError) and skips the chunk.

Acceptance Criteria

• examples/chat/README.md: remove the comment that a role-only first chunk "validates fine". CreateChatCompletionStreamResponse.model_validate raises ValidationError for a delta without content and for a choice whose finish_reason is null.
• examples/chat/README.md: catch pydantic.ValidationError alongside json.JSONDecodeError in the streaming snippet and skip the chunk, matching the root README.md "Streaming Content" example.
• examples/chat/main.py: apply the same ValidationError handling so python main.py does not crash on a chunk the strict model rejects.