Preserve Tooltip semantics when disabled by TooltipVisibility

[devzahirul]

TooltipVisibility(visible: false) is documented to "only visually" disable tooltips while it "continues to provide any semantic information that is provided". Since the RawTooltip refactor (#177678), TooltipState.build skips the RawTooltip wrapper entirely when the tooltip is disabled. Because the Semantics(tooltip:) annotation now lives inside RawTooltip, it is dropped along with it, so assistive technologies stop announcing the tooltip message — for example, an IconButton with tooltip: 'Add' under TooltipVisibility(visible: false) is announced as just "button". Before that refactor, the Semantics wrapper was applied unconditionally and only gesture handling was gated on visibility.

This change restores the documented behavior: when a TooltipVisibility ancestor disables the tooltip, the child is still wrapped in the same Semantics(tooltip:) annotation that RawTooltip applies when the tooltip is enabled, honoring excludeFromSemantics. It adds a regression test that fails without the fix, plus a test verifying that excludeFromSemantics: true still suppresses the semantics when the tooltip is disabled.

Fixes #189062

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[gemini-code-assist]

Code Review

This pull request updates the Tooltip widget to ensure that its message is still reported to assistive technology via semantics when a TooltipVisibility ancestor has disabled the tooltip overlay, unless excludeFromSemantics is set to true. It also adds corresponding regression tests to verify this behavior. There are no review comments to address.

[devzahirul]

This change targets the Material library, which is frozen in flutter/flutter per #184093. Closing in favor of the port to material_ui in flutter/packages, following the porting instructions in #188444: flutter/packages#12153