fix: prevent Calendar day cells from expanding

[KirtiRamchandani]

Previous Behavior

Calendar day cells could expand when an app-level table-cell padding rule had higher specificity than the default day-cell padding. For example, a scoped td { padding: 15px; } rule can turn the intended 28px cell into a 58px cell, which matches the DatePicker overflow reported in #36274.

New Behavior

Calendar day cells keep their fixed sizing by using border-box sizing and a higher-specificity zero-padding rule for the day-cell slot. This keeps the day grid at its expected width even when a surrounding app has scoped table-cell padding.

Related Issue(s)

• Fixes [Bug]: Expanded width of days of month in Date Picker #36274

Tests

• corepack yarn nx run react:test --testPathPatterns=CalendarDayGrid.test.tsx --runInBand --excludeTaskDependencies
• node node_modules\\eslint\\bin\\eslint.js packages\\react\\src\\components\\CalendarDayGrid\\CalendarDayGrid.styles.ts packages\\react\\src\\components\\CalendarDayGrid\\CalendarDayGrid.test.tsx
• node node_modules\\prettier\\bin-prettier.js --check packages\\react\\src\\components\\CalendarDayGrid\\CalendarDayGrid.styles.ts packages\\react\\src\\components\\CalendarDayGrid\\CalendarDayGrid.test.tsx change\\@fluentui-react-5e1a9878-e11f-4ae1-9cbf-71a0a7d296b9.json
• git diff --check
• Browser sanity check confirmed a 28px day cell under scoped external td { padding: 15px; }.