[material-ui][FormControlLabel] Add missing labelPlacementEnd class

[siriwatknp]

Summary

FormControlLabel composes a labelPlacement${capitalize(labelPlacement)} class key for every placement, but labelPlacementEnd was missing from formControlLabelClasses — so while start/top/bottom render their MuiFormControlLabel-labelPlacement* utility class on the root element, the default end placement renders none, and formControlLabelClasses.labelPlacementEnd doesn't exist as a constant for targeting.

This adds the missing labelPlacementEnd entry, so:

• the root element renders MuiFormControlLabel-labelPlacementEnd when labelPlacement="end" (including the default, when the prop is omitted) — consistent with the other three placements;
• formControlLabelClasses.labelPlacementEnd is available for styling and tests:

import { formControlLabelClasses } from '@mui/material/FormControlLabel';

const Styled = styled('div')({
  [`& .${formControlLabelClasses.labelPlacementEnd}`]: {
    marginRight: 8,
  },
});

No behavior or style change — the component already passed the key through composeClasses; it was silently dropped because no utility class was registered for it.

For Reviewers

• The class key composition in FormControlLabel.js is untouched — it already emits labelPlacementEnd for the default placement. The gap was only in the classes definition, so the fix is limited to registering the key in the interface + generateUtilityClasses list.
• Tests cover both the explicit labelPlacement="end" and the default (prop omitted) case, mirroring the existing start/top/bottom tests.
• API docs (form-control-label.json + translations) regenerated via docs:api:build.

---

• I have followed (at least) the PR section of the contributing guide.

[code-infra-dashboard]

Deploy preview

https://deploy-preview-48843--material-ui.netlify.app/

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	🔺+20B(0.00%)	🔺+7B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

---

Check out the code infra dashboard for more information about this PR.