fix(material/form-field): restore aria-live on mat-error

[charles-stivali-87]

Summary

Moves the live region for form-field errors back to mat-error instead of the persistent subscript wrapper. This prevents screen readers from announcing the same error twice when the control is already associated to the error message through aria-describedby.

Why

The current wrapper-level aria-live causes NVDA to announce the error once because the error content changes inside the live region, and again because the form control references the same error via aria-describedby.

By moving the live region back to mat-error, the announcement remains polite while avoiding the duplicate announcement caused by the always-present wrapper.

What changed

• restore aria-live="polite" on mat-error when no explicit value is provided
• keep aria-atomic="true" on the error message
• remove the wrapper-level live region from the subscript container
• update the input accessibility spec to assert the new behavior

Validation

• validated in a consumer app with NVDA
• confirmed the error is announced once instead of twice

Fixes #32104

cc: @wagnermaciel

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