Rename content to parts in gen_ai.input.messages

* We should emit best effort typed parts and, when required fields are unknown, fall back to `GenericPart` instead of emitting a typed part with missing required keys
* Clarify Strategy for Preserving `gen_ai.input.messages` History Between Model Calls
* The input list should include the most recent messages up to and including the most recent previous model response. The previous model response is identified with an "assistant" or "model" role in common frameworks. If there is no previous model response in the input list, then all input items which are not system instructions should be included. System instructions must be added in `gen_ai.system_instructions`, and are not included in the `gen_ai.input.messages` list.