Set `gen_ai.response.id` on `gen_ai.chat` spans

[ericapisani]

Extract response_id from pydantic-ai's ModelResponse and set it as gen_ai.response.id in update_ai_client_span() in sentry_sdk/integrations/pydantic_ai/spans/ai_client.py.

This is a recommended OTel attribute, useful for correlating spans with provider dashboards/logs. We already have SPANDATA.GEN_AI_RESPONSE_ID defined in consts.py.

[linear-code]

PY-2239