The taskdoubao adaptor expected snake_case JSON keys (video_url,
completion_tokens, total_tokens, framespersecond, service_tier,
created_at, updated_at) but Volcengine returns camelCase (videoUrl,
completionTokens, totalTokens, framesPerSecond, serviceTier,
createdAt, updatedAt). As a result the video URL was never parsed:
successful tasks fell back to the platform proxy URL
(BuildProxyURL) as result_url, which broke the video proxy in
containerized deployments (localhost:port not reachable inside the
container) and zeroed the usage tokens used for billing.

Align the responseTask JSON tags with the real Volcengine response
shape and add a regression test with the exact production payload.

Co-Authored-By: ZCode <noreply@anthropic.com>