Estimate run cost

Estimate what one run of a pipeline will charge the credit balance, which steps the money goes to (lines), and whether the current balance covers it. Estimates use the draft graph, the same one run_pipeline executes. When a video step would render the same clip for far less on Veo 3.1 Lite, cheaper names it with the run's price on it. When the balance covers neither, preview prices a first look it does cover: the first video step alone as a short Veo 3.1 Lite clip, which run_pipeline with preview: true renders. Call this before running a pipeline you built or changed in this conversation, tell the person the price (and the preview's, when there is one), and wait for their go-ahead before run_pipeline. Pass the returned quoteId to run_pipeline: it starts the run at this price once, and refuses if the pipeline changed since (estimate again then). Clients that show panels put the estimate in front of them with a Start render button, which sends that go-ahead as their message.