fix(openai-agents): properly serialize tool calls in output messages (open-telemetry#4185)

ResponseFunctionToolCall objects in response.output were being
stringified as text parts. Now detects tool call items by type
attribute and emits them as type:tool_call with id, name, and
arguments per GenAI semantic conventions.

Also handles ResponseOutputMessage objects whose content is a list
of sub-items rather than a plain string.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: nagkumar91