fix: Screenshot should complete or fail promptly after preview starts

[webstudio-issue-reporter]

User story

As a site editor, I need responsive screenshots before and after a narrow visual cleanup so I can verify that no layout changes occurred.

Summary

A shared MCP workflow starts an iterative preview successfully, then the screenshot call does not return a PNG or a structured error before the caller timeout.

What the agent tried

1. Start an iterative session preview.
2. Capture a full-page root-route screenshot with a desktop viewport and explicit readiness and timeout settings.

Expected behavior

The screenshot should return a PNG artifact and metrics, or return a structured timeout within the requested bound.

Actual result

Preview start succeeded. Screenshot remained pending until the outer caller timed out after about one minute, with no artifact or tool result.

Recovery attempts

1. Used a shared multi-call MCP session so preview ownership and screenshot ownership match.
2. Corrected the workflow call input nesting after validation feedback and retried once.

User impact

A planned, approved CSS cleanup cannot proceed because the required before-and-after responsive visual verification is unavailable.

Technical context

The failure occurs in the MCP visual workflow after preview.start and during screenshot with path, viewport, fullPage, readiness, and timeout fields.

Agent environment

• Client: Codex
• Provider: OpenAI
• Model: gpt-5.6-sol
• Reasoning effort: medium
• Trigger: automatic-friction
• Category: hang-or-crash

Technical runtime

• CLI: 0.287.0
• Node.js: v26.2.0
• Operating system: Windows unknown (x64)
• Execution mode: mcp
• API contract: public-api:iqd6ok

Acceptance criteria

1. Screenshot returns a PNG or a structured timeout for this workflow.
2. The multi-call preview-start then screenshot path has automated coverage.
3. A failed screenshot leaves no orphaned preview process or session lock.