A native function/tool call whose `arguments` field is valid JSON but not an
object — a bare array like ["ls -la"], or a string/number/bool/null — parsed
fine in function_call_to_tool_block and then every branch called args.get(...),
raising AttributeError ('list'/'str' object has no attribute 'get'). That
propagated out of the streamed agent loop (no surrounding try/except at the
call site in stream_agent_loop) and aborted the user's entire turn. Weaker and
local models routinely emit malformed args like this.
Coerce non-dict parsed arguments to {} (mirrors the existing empty-arguments
behavior), so the tool runs with empty args instead of killing the stream.
Adds tests/test_function_call_non_object_args.py covering array/string/number/
bool/null arguments — they fail before this change and pass after.
