fix(streaming): scope role-only chunk preservation to Azure only

The Azure streaming fix (PR #25638) made is_chunk_non_empty() treat
any first chunk with role!=None as non-empty, for all providers. This
broke OpenAI/Mistral tests where the role-only first chunk was expected
to be filtered, and test idx==0 assumed the first emitted chunk had
actual content (tool_calls, logprobs, etc.).

Restrict the condition to Azure, which is the provider that actually
needs it: Azure sends prompt_filter_results before the first content
chunk, which consumes sent_first_chunk and strips role from the real
first chunk. Other providers should retain the original behavior.

litellm/litellm_core_utils/streaming_handler.py

@@ -839,10 +839,7 @@ class CustomStreamWrapper:
                 not self.sent_first_chunk
                 and hasattr(model_response.choices[0].delta, "role")
                 and model_response.choices[0].delta.role is not None
-                and not (
-                    model_response.choices[0].logprobs is not None
-                    and not model_response.choices[0].logprobs.content
-                )
+                and self.custom_llm_provider == "azure"
             )
         ):
             return True