Fix #7157 - Add None guards on tool_call_chunk fields in Azure AI streaming

[Whning0513]

AzureAIChatCompletionClient streaming tool_calls can include None fields, causing TypeError

When using AzureAIChatCompletionClient with GitHub Models endpoint (https://models.inference.ai.azure.com) and streaming with tool calls enabled, the streaming tool-call aggregation crashes with TypeError: can only concatenate str (not "NoneType") to str because tool_call_chunk.id, tool_call_chunk.function.name, or tool_call_chunk.function.arguments can be None in streamed deltas.

Reported by: daniel-hou-tfs

The Azure AI Inference SDK may emit streaming tool call chunks where individual fields (id, function.name, function.arguments) are None in certain deltas. The original code unconditionally concatenated these fields using +=, which raises TypeError when the right-hand side is None.

File: autogen/python/packages/autogen-ext/src/autogen_ext/models/azure/_azure_ai_client.py

Added None guards before concatenation in the streaming tool-call aggregation loop (around line 550):

1. if tool_call_chunk.id is not None: before full_tool_calls[idx].id += tool_call_chunk.id
2. if tool_call_chunk.function is not None: check for function field existence
3. if tool_call_chunk.function.name is not None: before name concatenation
4. if tool_call_chunk.function.arguments is not None: before arguments concatenation

This mirrors the pattern already used in the OpenAI client for the same purpose.

Low - The change adds defensive None checks in the streaming aggregation path. It prevents crashes that would otherwise occur with certain API responses and does not modify any existing valid behavior.

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