fix(chatcompletions): repair truncated tool-call args before emission

A streaming tool call cut off after the opening brace would leave
state.function_calls[i].arguments = "{". CAI persisted that into
conversation history verbatim, and on the next turn the upstream
litellm proxy strict-parsed it as JSON and rejected the whole
request with HTTP 400 ("unexpected end of data: line 1 column 2"),
wedging the session.

- openai_chatcompletions.py: at end-of-stream, validate every
  accumulated function_call.arguments string. If empty or not
  valid JSON, normalize to "{}" before emitting events.
- message_builder.py: same guard when replaying assistant tool_calls
  out of memory, so a poisoned history loaded from disk also recovers.

src/cai/sdk/agents/models/chatcompletions/message_builder.py

@@ -424,6 +424,14 @@ class Converter:
                         arguments = "{}"
                     elif isinstance(arguments, dict):
                         arguments = json.dumps(arguments)
+                    else:
+                        # Truncated/streamed-then-cut function call args (e.g. "{")
+                        # are not valid JSON and the upstream proxy rejects the whole
+                        # request with HTTP 400, wedging the conversation.
+                        try:
+                            json.loads(arguments)
+                        except (TypeError, ValueError):
+                            arguments = "{}"
                     tool_calls_param.append(
                         ChatCompletionMessageToolCallParam(
                             id=tc.get("id", "")[:40],

src/cai/sdk/agents/models/openai_chatcompletions.py

@@ -2676,6 +2676,19 @@ class OpenAIChatCompletionsModel(Model):
                         type="response.content_part.done",
                     )
 
+                # Repair partially-streamed function calls before emission so a
+                # truncated args string like "{" cannot poison conversation history
+                # and trigger HTTP 400 on the next request.
+                for _fc in state.function_calls.values():
+                    _args = _fc.arguments or ""
+                    if not _args.strip():
+                        _fc.arguments = "{}"
+                    else:
+                        try:
+                            json.loads(_args)
+                        except (TypeError, ValueError):
+                            _fc.arguments = "{}"
+
                 # Actually send events for the function calls
                 for function_call in state.function_calls.values():
                     # First, a ResponseOutputItemAdded for the function call