fix(ui5-media-gallery): acc findings fixed

[PetyaMarkovaBogdanova]

4) MediaGallery (22 failing)

• Repeated continuum violations, including:
• LI with role="option" missing aria-checked (ID 1079).
• UL child-structure violation for list semantics (ID 246).
• Failing examples include thumbnail options and overflow scenarios.

Component	Issue	Not by spec	Good to have	Spec refs	Spec quote sentence	Coverage strength
MediaGallery	role=option missing required state	Likely Yes	No	ACC-264	"Item in data set has ARIA role=option" and "Selected item should be indicated using aria-selected=true"	Indirect (pattern-based)
MediaGallery	UL child-structure/list semantics violation	Yes	No	ACC-263	"The structure should be an unordered list containing anchors and separators inside list items"	Indirect (list-structure pattern)

[ui5-webcomponents-bot]

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[yanaminkova]

There is no aria-selected attribute, the correct one is aria-checked.

Adding role="listbox" to the <ul> resolves the current validation error, but it introduces another one related to the overflow button. To satisfy the accessibility requirements, the element with the ui5-media-gallery-overflow class should also have role="option" and an aria-checked attribute.

Could you also please add Cypress tests?