Generates a PDF/HTML rendering of a proposal's layout. Requires the company's layout_template policy to be enabled, the estimate must have is_proposal: true, and have a layout association configured. Runs fully synchronously (PDF generation + upload happen in-request, no job queue).

If the proposal has already reached APPROVED or AWAIT_SIGNATURE, only proposal_options marked is_accepted: true are rendered (post-approval, present shows only the winning option).

send_buffer: true bypasses the normal JSON envelope entirely and returns raw HTML directly.