fix(cli): improve smoke test image validation with 404 checks and filepath detection

[thesandlord]

Description

Replaces the unreliable naturalWidth > 0 check in the docs preview smoke test with more robust image validation that will actually catch broken images on Windows.

Changes Made

• Network-level 404 detection: Intercepts all HTTP responses and requestfailed events during page load to catch image requests that return 4xx/5xx or fail at the network level
• Filesystem path pattern detection: Extracts looksLikeFilesystemPath() helper that checks for:
  • file:/// protocol leaks
  • Windows drive letters (C:/, D:\)
  • URL-encoded backslashes (%5C)
  • Raw backslashes
  • Unresolved file:<uuid> references (frontend failed to map to a URL)
• Removed naturalWidth check which could pass even when images fail to render correctly (e.g., browser placeholder icons have non-zero dimensions)
• Changed waitUntil from "domcontentloaded" to "load" to ensure images have had time to start loading before we check network responses

Testing

• Manual review of test logic
• The smoke test CI workflow runs on ubuntu-latest, macos-latest, and windows-latest — the improved assertions will surface any image resolution failures that the old naturalWidth check was masking

Link to Devin session: https://app.devin.ai/sessions/33e519fd68c24507b26161112fbb6d74
Requested by: @thesandlord

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