Replace deprecated `gen_ai.prompt` with `gen_ai.input.messages`

`@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`]()

Start in the Anthropic AI integration and review the linked gen_ai.prompt and gen_ai.input.messages convention references. Confirm how the legacy raw prompt is currently emitted, then verify that the structured message attribute contains the wrapped prompt and that the deprecated attribute is no longer emitted. Relay handling and the major-release timing remain open questions in the issue.

Written by the indexing model from the issue text.