A user turn's `content` may now be a part array (`ChatTextPart` +
`ChatImagePart`), which chat completions take as-is; the Anthropic and
Responses bridges translate it into `image` blocks and `input_image` parts.
A tool result keeps its text in `content` and carries `images` beside it:
Anthropic and Responses put them in the result item itself (a live probe reads
both back), while chat completions cannot, so
`compat.chat_messages_with_images` moves them into the user turn after the tool
run. Prose-only readers go through `content_to_text`, which renders an image as
`[image]`.
`agent.images` sniffs the real format from the bytes and builds the data URL,
capped at 8 MiB inline because every stored message is re-sent on each round.
`provider_blocks` means something only to the surface that produced them (Anthropic `thinking` / server-tool blocks). A session that switched surface would otherwise send those dicts to a chat-completions provider as an assistant-message key it does not know; the request normalization drops the field, on a copy, so the history keeps it.
NCBM