[material_ui] Add supportingTextPadding to InputDecorator

[MohanadAbdallah-mv]

This PR introduces the supportingTextPadding property to InputDecorator, InputDecoration, and InputDecorationThemeData.
While originally conceived as errorPadding, this property was renamed to supportingTextPadding during review to align with the Material Design 3 specification, as it controls the padding for the entire supporting text row (which includes helper text, error text, and counter widgets).
Additionally, this PR addresses layout, intrinsic sizing, and code consistency concerns raised during the review process:

Key Features & Refactors:

1. Added supportingTextPadding: Allows complete, independent control over the padding of the supporting text row (helper/error/counter).
2. Decoupled Layout Constraints: Introduced supportingTextConstraints in _layout() to bound the supporting text row independently of contentConstraints, ensuring custom paddings do not break or shift the layout of inline prefix/suffix widgets.
3. Fixed Intrinsic Height Mismatches: Corrected computeMinIntrinsicHeight() to compute supportingTextWidth separately from the primary width deflation path, ensuring intrinsic height calculations accurately predict wrapped text heights.
4. Unified Terminology: Renamed all legacy internal subtext references (e.g. _SubtextSize, _computeSubtextSizes, subtextHeight) to supportingText to match the public property name and the M3 spec.

List which issues are fixed by this PR. You must list at least one issue.
Port of #183582 which Fixes #175834

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[gemini-code-assist]

Code Review

This pull request introduces the supportingTextPadding property to InputDecorator, InputDecoration, InputDecorationTheme, and InputDecorationThemeData to allow custom padding for the supporting text row, while renaming internal 'subtext' references to 'supportingText'. Feedback on these changes highlights that InputDecorationThemeData.lerp is missing interpolation for the new supportingTextPadding property, which is necessary for smooth theme transitions, and suggests adding a corresponding test case to verify this behavior.

[gemini-code-assist]

The InputDecorationThemeData.lerp method is missing the interpolation for supportingTextPadding. Without this, transitions between different input decoration themes will not animate the supporting text padding smoothly.

Please update InputDecorationThemeData.lerp to include:

supportingTextPadding: EdgeInsetsGeometry.lerp(a?.supportingTextPadding, b?.supportingTextPadding, t),

[gemini-code-assist]

Consider adding a test case to verify that InputDecorationThemeData.lerp correctly interpolates supportingTextPadding between two themes. This ensures that theme animations work as expected and prevents future regressions.

[MohanadAbdallah-mv]

@dkwingsmt @Renzo-Olivares
This current PR is a port of #183582.