fix(react-button): make SplitButton divider match Figma spec

[mainframev]

Current SplitButton subtle and transparent variants have a divider that does not match Figma specification.

Previous Behavior

New Behavior

Related Issue(s)

• Fixes [Bug]: Fix Split button styling issue #33721

[github-actions]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
react-button SplitButton	50.032 kB 13.724 kB	50.08 kB 13.724 kB	48 B
react-components react-components: entire library	1.169 MB 292.741 kB	1.169 MB 292.746 kB	48 B 5 B

Unchanged fixtures

Package & Exports	Size (minified/GZIP)
react-breadcrumb @fluentui/react-breadcrumb - package	114.317 kB 31.702 kB
react-button Button	37.2 kB 10.811 kB
react-button CompoundButton	43.614 kB 12.109 kB
react-button MenuButton	42.015 kB 12.152 kB
react-button ToggleButton	53.133 kB 12.569 kB
react-card Card - All	101.796 kB 28.779 kB
react-card Card	94.57 kB 26.957 kB
react-card CardFooter	14.381 kB 5.799 kB
react-card CardHeader	16.914 kB 6.677 kB
react-card CardPreview	14.446 kB 5.929 kB
react-components react-components: Button, FluentProvider & webLightTheme	69.236 kB 20.182 kB
react-components react-components: Accordion, Button, FluentProvider, Image, Menu, Popover	224.18 kB 64.859 kB
react-components react-components: FluentProvider & webLightTheme	44.473 kB 14.597 kB
react-dialog Dialog (including children components)	100.513 kB 30.131 kB
react-message-bar MessageBar (all components)	24.851 kB 9.276 kB
react-portal-compat PortalCompatProvider	8.39 kB 2.64 kB
react-tag-picker @fluentui/react-tag-picker - package	185.734 kB 55.759 kB
react-teaching-popover TeachingPopover	91.711 kB 27.921 kB
react-timepicker-compat TimePicker	108.551 kB 36.094 kB
react-tree FlatTree	147.295 kB 42.271 kB
react-tree PersonaFlatTree	148.04 kB 42.417 kB
react-tree PersonaTree	144.249 kB 41.27 kB
react-tree Tree	143.51 kB 41.149 kB

_{🤖 This report was generated against 87d6ba3bd577a8be9756f4d392b048083d8ef16a}

[github-actions]

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