Make structural accessibility-sweep skips explicit

[Blackspirits]

Summary

The accessibility sweep previously allowed any window to disappear from coverage when it lacked a single-argument constructor or its constructor dependency was not registered in DI. The only aggregate guard was opened > 50, so future windows could silently become untested.

The audit probe made all structural skips fail and exposed exactly two deterministic exclusions on both the initial run and retry:

• FullScreenVideoWindow: a runtime-only fullscreen host that requires an existing VideoPlayerControl, media state and callbacks; it is not a DI/view-model tool window;
• MessageBox: a factory-created modal with a private multi-argument constructor, exercised through MessageBox.Show rather than DI construction.

This revision:

• names those two legitimate exclusions explicitly in SkippedWindows with reasons;
• keeps skipped.Count == 0 after explicit exclusions;
• therefore makes any future undeclared structural skip fail CI instead of silently reducing coverage;
• changes no production code.

Validation

• based on upstream 40c97b2def70f1c7b919602fbf4e7bed7d0c9fd3
• probe CI #34771411574 failed on exactly FullScreenVideoWindow and MessageBox
• retry reproduced the exact same two exclusions
• revised full solution restore passed
• revised full solution build passed
• revised full solution test suite passed on the first run
• UI suite: 5,149 passed, 9 skipped, 0 failed (5,158 total)
• retry step was not used
• validation run: #34772761754

AI assistance: ChatGPT was used to audit fail-open structural coverage, classify the two existing exclusions, and convert implicit skipping into an explicit allowlist contract.

[Blackspirits]

Independent adversarial re-check: the zero-skip probe deterministically exposed only FullScreenVideoWindow and MessageBox on both the initial run and retry. Both are structurally outside the DI/view-model sweep contract, and the final revision names only those two exclusions while preserving skipped.Count == 0 for every undeclared future skip. Full CI #34772761754 passed restore, build and the complete suite on the first run; UITests reported 5,149 passed, 9 skipped and 0 failed. Retry was not used. No blocker identified. Keep draft; no merge performed.