Replace deprecated gen_ai.prompt with gen_ai.input.messages #22837

@sentry/conventions deprecates gen_ai.prompt in favor of gen_ai.input.messages. gen_ai.prompt holds a raw prompt string (legacy Anthropic Completions API); gen_ai.input.messages expects a stringified array of structured message objects ([{role, parts: [{type, content}]}]).

Entails wrapping the legacy prompt into the new structured message format and emitting it under the new attribute. Affects the Anthropic AI integration.

We can likely postpone this until after the major, if we do it at all, since there are still discussions about handling this in Relay.

References:

• gen_ai.prompt
• gen_ai.input.messages