feat: attachment and content filter error kinds

[Im-Beast]

• Adds invalid_attachment for files we fail to fetch or parse ourselves, never retried
• Adds attachment_rejected for a provider refusing the file, switches model and skips it for the run
• Adds content_filtered for guardrail rejections, never retried
• Attachment fetches now fail on non-2xx and empty bodies, including the Anthropic text fetch that sent a 404 body as file text
• Remaps overloaded_error, terminated, "had an error processing your request" and "missing finish_reason" onto transient kinds
• Bumps to 0.7.1