Add a way to delete message parts and messages in the sdk #3033

My use case is writing a plugin that removes tool parts from plan messages, this way I can switch from GPT5 to Sonnet after a plan message without getting the model confused by all the tool calls in context

ChatGPT already does this (it exposes previous message reasoning via an on demand tool), follow up messages discard all the reasoning parts and only keep the final message in context. Codex may do the same too.