Capture `gen_ai.output.type` on `invoke_agent` spans

[ericapisani]

Set gen_ai.output.type on invoke_agent spans to indicate whether the agent output is text or json (structured output).

Pydantic-ai's structured output is a key feature — this attribute (defined by OTel with values text, json, image, speech) would help users understand what kind of output was produced. Can be inferred from the agent's output_type configuration.

[linear-code]

PY-2243

[alexander-alderman-webb]

not defined by getsentry/rfcs#153