Cleanup InputDecoration.collapsed constructor (flutter#152165)

## Description

This PR attemps to clarify the `InputDecoration.collapsed` documentation by explaining that it is not meant to be use with helper, label, counter, icons, prefixes and suffixes.

It also adds some parameters that make sense for a collapsed decoration (`hintMaxLines`, `hintFadeDuration`, `constraints`).

Removing parameters that should not have been added (`floatingLabelBehavior` and `floatingLabelAlignment`) will be part of another PR as it will require deprecations.

## Related Issue

Fixes flutter#61331

## Tests

Adds 3 tests.

Author: bleroux

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

@@ -2622,19 +2622,26 @@ class InputDecoration {
   ///
   /// This type of input decoration does not include a border by default.
   ///
+  /// A collapsed decoration cannot have [labelText], [errorText], [counter],
+  /// [icon], prefixes, and suffixes.
+  ///
   /// Sets the [isCollapsed] property to true.
+  /// Sets the [contentPadding] property to [EdgeInsets.zero].
   const InputDecoration.collapsed({
     required this.hintText,
     this.floatingLabelBehavior,
     this.floatingLabelAlignment,
     this.hintStyle,
     this.hintTextDirection,
+    this.hintMaxLines,
+    this.hintFadeDuration,
     this.filled = false,
     this.fillColor,
     this.focusColor,
     this.hoverColor,
     this.border = InputBorder.none,
     this.enabled = true,
+    this.constraints,
   }) : icon = null,
        iconColor = null,
        label = null,
@@ -2645,8 +2652,6 @@ class InputDecoration {
        helperText = null,
        helperStyle = null,
        helperMaxLines = null,
-       hintMaxLines = null,
-       hintFadeDuration = null,
        error = null,
        errorText = null,
        errorStyle = null,
@@ -2675,8 +2680,7 @@ class InputDecoration {
        disabledBorder = null,
        enabledBorder = null,
        semanticCounterText = null,
-       alignLabelWithHint = false,
-       constraints = null;
+       alignLabelWithHint = false;
 
   /// An icon to show before the input field and outside of the decoration's
   /// container.
@@ -3019,7 +3023,8 @@ class InputDecoration {
 
   /// Whether the decoration is the same size as the input field.
   ///
-  /// A collapsed decoration cannot have [labelText], [errorText], an [icon].
+  /// A collapsed decoration cannot have [labelText], [errorText], [counter],
+  /// [icon], prefixes, and suffixes.
   ///
   /// To create a collapsed input decoration, use [InputDecoration.collapsed].
   final bool? isCollapsed;
@@ -3881,7 +3886,7 @@ class InputDecoration {
 /// the current input decoration theme to initialize null [InputDecoration]
 /// properties.
 ///
-/// The [InputDecoration.applyDefaults] method is used to combine a input
+/// The [InputDecoration.applyDefaults] method is used to combine an input
 /// decoration theme with an [InputDecoration] object.
 @immutable
 class InputDecorationTheme with Diagnosticable {

packages/flutter/test/material/input_decorator_test.dart

@@ -6523,6 +6523,70 @@ void main() {
       expect(getBorderWeight(tester), 0.0);
     });
 
+    testWidgets('InputDecoration.collapsed accepts constraints', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        buildInputDecorator(
+          decoration: const InputDecoration.collapsed(
+            hintText: hintText,
+            constraints: BoxConstraints.tightFor(width: 200.0, height: 32.0),
+          ),
+        ),
+      );
+
+      expect(getDecoratorRect(tester).size, const Size(200.0, 32.0));
+    });
+
+    testWidgets('InputDecoration.collapsed accepts hintMaxLines', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        buildInputDecorator(
+          decoration: const InputDecoration.collapsed(
+            hintText: threeLines,
+            hintMaxLines: 2,
+          ),
+        ),
+      );
+
+      const double hintLineHeight = 24.0; // font size = 16 and font height = 1.5.
+      expect(getDecoratorRect(tester).size, const Size(800.0, 2 * hintLineHeight));
+    });
+
+    testWidgets('InputDecoration.collapsed accepts hintFadeDuration', (WidgetTester tester) async {
+      // Build once with empty content.
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          decoration: const InputDecoration.collapsed(
+            hintText: hintText,
+            hintFadeDuration: Duration(milliseconds: 120),
+          ),
+        ),
+      );
+
+      // Hint is visible (opacity 1.0).
+      expect(getHintOpacity(tester), 1.0);
+
+      // Rebuild with non-empty content.
+      await tester.pumpWidget(
+        buildInputDecorator(
+          decoration: const InputDecoration.collapsed(
+            hintText: hintText,
+            hintFadeDuration: Duration(milliseconds: 120),
+          ),
+        ),
+      );
+
+      // The hint's opacity animates from 1.0 to 0.0.
+      // The animation's default duration is 20ms.
+      await tester.pump(const Duration(milliseconds: 50));
+      final double hintOpacity50ms = getHintOpacity(tester);
+      expect(hintOpacity50ms, inExclusiveRange(0.0, 1.0));
+      await tester.pump(const Duration(milliseconds: 50));
+      final double hintOpacity100ms = getHintOpacity(tester);
+      expect(hintOpacity100ms, inExclusiveRange(0.0, hintOpacity50ms));
+      await tester.pump(const Duration(milliseconds: 50));
+      expect(getHintOpacity(tester), 0.0);
+    });
+
     test('InputDecorationTheme.isCollapsed is applied', () {
       final InputDecoration decoration = const InputDecoration(
         hintText: 'Hello, Flutter!',