[input base] Move multiline padding to the textarea

[yiheng-kkk]

Summary

• move the built-in multiline padding from the InputBase, FilledInput, and OutlinedInput roots to the native textarea
• make the textarea own the padding hit area so Firefox can place the caret nearest to the click
• preserve the existing padding and dimensions across variants, sizes, hidden labels, adornments, and autosizing scroll

Fixes #23928

Validation

• focused Firefox browser regressions: 3 passed

• relevant Firefox browser suites: 121 passed, 3 skipped

• relevant Node suites: 100 passed, 24 skipped

• TypeScript, ESLint, Prettier, and git diff --check: passed

• interactive Firefox RED → GREEN: clicking the upper-left padding kept the caret at offset 22 before the fix and moved it to offset 0 after the fix

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

AI assistance disclosure: this PR was prepared with Codex assistance; the final diff and validation outputs were reviewed before submission.

[code-infra-dashboard]

Deploy preview

https://deploy-preview-49020--material-ui.netlify.app/

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	▼-849B(-0.16%)	▼-125B(-0.08%)
@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

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