Replace window.sendRuntimeMessage existence check with URL-based detection.
The sendRuntimeMessage function exists in both extension and non-extension
contexts (e.g., downloaded HTML artifacts), causing incorrect behavior.
Changes:
- Add window.__isExtensionContext() helper to RuntimeMessageBridge that checks:
- chrome-extension:// URLs (Chrome)
- moz-extension:// URLs (Firefox)
- about:srcdoc (sandbox iframes)
- Update all runtime providers to use __isExtensionContext() instead:
- FileDownloadRuntimeProvider: Correctly falls back to browser download
- ConsoleRuntimeProvider: Only sends messages in extension context
- ArtifactsRuntimeProvider: Properly detects offline/read-only mode
This fixes the issue where downloaded HTML artifacts incorrectly try to
communicate with the extension when opened from disk.
b5648eaabd