[wpt-gen vendoring] Update WPTGenEngine for filesystem-optional mode

[szy196]

Background & Motivation

In server-side environments like chromium-dashboard, a local checkout of the Web Platform Tests (WPT) repository may not be available. The engine must be updated to handle wpt_path=None and rely on external metadata.

Part of #439.

Proposed Changes

• Modify WPTGenEngine and run_context_assembly to handle a None wpt_path.
• Update find_feature_tests and gather_local_test_context to skip local scanning when the path is missing.
• Adjust the Coverage Audit phase to use only ChromeStatus metadata and explainer/spec links when in filesystem-optional mode.

Acceptance Criteria

• WPTGenEngine runs successfully when config.wpt_path is None.
• Context assembly correctly identifies and reports missing local test context.
• The coverage audit proceeds using available external context.

_{This issue description was drafted by Gemini}