Do not treat empty visual text as an accessibility label

[Blackspirits]

Summary

AccessibleLabels.IsLabel currently accepts empty visual controls as inferred labels:

• a TextBlock with Text == null;
• a Label with null/empty string content;
• a CheckBox / RadioButton with an empty string.

Once one of those controls is assigned through AutomationProperties.LabeledBy, HasAccessibleName considers the input named solely because the reference is non-null. That creates a false-green path where a literal empty spacer/status control can satisfy the accessibility sweep.

The first stricter CI run exposed five real places that relied on an initially-empty dynamic label. Rather than weakening the inference rule again, this revision makes those intended relationships explicit:

• Manual chosen encoding preview → its bound current-encoding label;
• Edit whole text → its bound line-info label;
• Apply duration limits → each table → its bound status/section label;
• Pick audio track → stable “Select audio track” accessible name.

This change therefore:

• requires non-whitespace visible text before Label, TextBlock, CheckBox or RadioButton may be used as an inferred label;
• preserves explicit LabeledBy relationships even when their bound text is temporarily empty;
• adds focused regressions for empty TextBlock, empty Label, and a normal short text label;
• converts the five CI-discovered dynamic cases from heuristic inference to explicit semantics.

Validation

• based on upstream 40c97b2def70f1c7b919602fbf4e7bed7d0c9fd3
• initial CI #34767587845 failed deterministically in AccessibleNamesTests and identified the five dynamic-label dependencies above; retry reproduced the same five failures
• revised full solution restore passed
• revised full solution build passed
• revised full solution test suite passed on the first run
• UI suite: 5,152 passed, 9 skipped, 0 failed (5,161 total)
• retry step was not used
• validation run: #34768068663

AI assistance: ChatGPT was used to adversarially review inferred accessibility-label semantics, use the full accessibility sweep to discover dynamic-label dependencies, and replace heuristic empty-label acceptance with explicit relationships.

[Blackspirits]

Independent adversarial re-check: the stricter inferred-label rule no longer accepts empty visual text, while the five dynamic-label dependencies exposed by the first CI run are now explicit relationships rather than heuristic inference. The final full-solution CI #34768068663 passed restore, build and all tests on the first run; UITests reported 5,152 passed, 9 skipped, 0 failed. Retry was not used. No blocker identified. Keep draft; no merge or upstream change performed.