Fix TextField helper top padding on M3 (#145087)

## Description

`InputDecorator` adds a 8.0 gap beetween the container and the helper text.
From the Material 3 specification, this gap should be 4.0.
See https://m3.material.io/components/text-fields/specs#0c5c8d6d-2169-4d42-960c-51f6ee42eb57.

This PR sets the correct values for M3 without changing the value for M2.

| Before | After | M3 Spec |
|--------|--------|--------|
| ![image](https://github.com/flutter/flutter/assets/840911/9947f334-d98f-4f7e-9da7-ca6d5c0770ac) | ![image](https://github.com/flutter/flutter/assets/840911/081dec4b-eb9f-4eee-a7dc-2538e7110ff0)| ![image](https://github.com/flutter/flutter/assets/840911/c8c8f045-3b79-43a5-a1a3-cc6d5460644f) |

If this change is accepted, a future step will be to make this value configurable, probably by `InputDecorationTheme`.

## Related Issue

Fixes flutter/flutter#144984

## Tests

Updates a value used by several existing tests.

Author: bleroux

packages/flutter/lib/src/material/input_decorator.dart

@@ -726,7 +726,9 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
        _expands = expands,
        _material3 = material3;
 
-  static const double subtextGap = 8.0;
+  // TODO(bleroux): consider defining this value as a Material token and making it
+  // configurable by InputDecorationTheme.
+  double get subtextGap => material3 ? 4.0 : 8.0;
 
   RenderBox? get icon => childForSlot(_DecorationSlot.icon);
   RenderBox? get input => childForSlot(_DecorationSlot.input);

packages/flutter/test/material/input_decorator_test.dart

@@ -1504,23 +1504,20 @@ void main() {
   });
 
   group('Material3 - InputDecoration helper/counter/error', () {
-    // Overall height for InputDecorator (filled or outlined) is 80dp on mobile:
+    // Overall height for InputDecorator (filled or outlined) is 76dp on mobile:
     //    8 - top padding
     //   12 - floating label (font size = 16 * 0.75, line height is forced to 1.0)
     //    4 - gap between label and input
     //   24 - input text (font size = 16, line height = 1.5)
     //    8 - bottom padding
-    //    8 - gap above supporting text
+    //    4 - gap above helper/error/counter
     //   16 - helper/counter (font size = 12, line height is 1.5)
     const double topPadding = 8.0;
     const double floatingLabelHeight = 12.0;
     const double labelInputGap = 4.0;
     const double inputHeight = 24.0;
     const double bottomPadding = 8.0;
-    // TODO(bleroux): make the InputDecorator implementation compliant with M3 spec by changing
-    // the helperGap to 4.0 instead of 8.0.
-    // See https://github.com/flutter/flutter/issues/144984.
-    const double helperGap = 8.0;
+    const double helperGap = 4.0;
     const double helperHeight = 16.0;
     const double containerHeight = topPadding + floatingLabelHeight + labelInputGap + inputHeight + bottomPadding; // 56.0
     const double fullHeight = containerHeight + helperGap + helperHeight; // 80.0 (should be 76.0 based on M3 spec)