0358f5736d80cf0f3a62e3651484c4cf6b96dc50  AX: the value of a dropdown box ignores an author <button> and the number of selected options

Classified as not security-relevant.
Triage note: Extracts a shared HTMLSelectElement::buttonText(ForAccessibility) used by both AccessibilityRenderObject::stringValue and SelectFallbackButtonElement, and adds deferMenuListValueChange AX notification. Refactor/accessibility behavior, no security bug.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), null_guard (+2.59), area:Source/WebCore/html (+2.0)