feat(Tooltip): pass the default text style down the tree (#163259)

Modify the tooltip's DefaultTextStyle to use the actual default text
style picked to fit the current theme and platform instead of hardcoding
the bodyMedium text style.

Closes: #163255

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responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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

@@ -191,12 +191,6 @@ class Tooltip extends StatefulWidget {
   }) : assert(
          (message == null) != (richMessage == null),
          'Either `message` or `richMessage` must be specified',
-       ),
-       assert(
-         richMessage == null || textStyle == null,
-         'If `richMessage` is specified, `textStyle` will have no effect. '
-         'If you wish to provide a `textStyle` for a rich tooltip, add the '
-         '`textStyle` directly to the `richMessage` InlineSpan.',
        );
 
   /// The text to display in the tooltip.
@@ -978,8 +972,8 @@ class _TooltipOverlay extends StatelessWidget {
     this.padding,
     this.margin,
     this.decoration,
-    this.textStyle,
-    this.textAlign,
+    required this.textStyle,
+    required this.textAlign,
     required this.animation,
     required this.target,
     required this.verticalOffset,
@@ -994,8 +988,8 @@ class _TooltipOverlay extends StatelessWidget {
   final EdgeInsetsGeometry? padding;
   final EdgeInsetsGeometry? margin;
   final Decoration? decoration;
-  final TextStyle? textStyle;
-  final TextAlign? textAlign;
+  final TextStyle textStyle;
+  final TextAlign textAlign;
   final Animation<double> animation;
   final Offset target;
   final double verticalOffset;
@@ -1011,7 +1005,8 @@ class _TooltipOverlay extends StatelessWidget {
       child: ConstrainedBox(
         constraints: BoxConstraints(minHeight: height),
         child: DefaultTextStyle(
-          style: Theme.of(context).textTheme.bodyMedium!,
+          style: textStyle,
+          textAlign: textAlign,
           child: Semantics(
             container: true,
             child: Container(

packages/flutter/test/material/tooltip_test.dart

@@ -3390,6 +3390,65 @@ void main() {
 
     await gesture.removePointer();
   });
+
+  testWidgets('Tooltip should pass its default text style down to widget spans', (
+    WidgetTester tester,
+  ) async {
+    final GlobalKey<TooltipState> tooltipKey = GlobalKey<TooltipState>();
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Tooltip(
+          key: tooltipKey,
+          richMessage: const WidgetSpan(child: Text(tooltipText)),
+          child: Container(width: 100.0, height: 100.0, color: Colors.green[500]),
+        ),
+      ),
+    );
+    tooltipKey.currentState?.ensureTooltipVisible();
+    await tester.pump(const Duration(seconds: 2));
+
+    final Finder defaultTextStyle = find.ancestor(
+      of: find.text(tooltipText),
+      matching: find.byType(DefaultTextStyle),
+    );
+    final DefaultTextStyle textStyle = tester.widget<DefaultTextStyle>(defaultTextStyle.first);
+    expect(textStyle.style.color, Colors.white);
+    expect(textStyle.style.fontFamily, 'Roboto');
+    expect(textStyle.style.decoration, TextDecoration.none);
+    expect(
+      textStyle.style.debugLabel,
+      '((englishLike bodyMedium 2021).merge((blackMountainView bodyMedium).apply)).copyWith',
+    );
+  });
+
+  testWidgets('Tooltip should apply provided text style to rich messages', (
+    WidgetTester tester,
+  ) async {
+    final GlobalKey<TooltipState> tooltipKey = GlobalKey<TooltipState>();
+    const TextStyle expectedTextStyle = TextStyle(color: Colors.orange);
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Tooltip(
+          key: tooltipKey,
+          richMessage: const TextSpan(text: tooltipText),
+          textStyle: expectedTextStyle,
+          child: Container(width: 100.0, height: 100.0, color: Colors.green[500]),
+        ),
+      ),
+    );
+    tooltipKey.currentState?.ensureTooltipVisible();
+    await tester.pump(const Duration(seconds: 2));
+
+    final TextStyle textStyle = tester.widget<Text>(find.text(tooltipText)).style!;
+    final Finder defaultTextStyleFinder = find.ancestor(
+      of: find.text(tooltipText),
+      matching: find.byType(DefaultTextStyle),
+    );
+    final TextStyle defaultTextStyle =
+        tester.widget<DefaultTextStyle>(defaultTextStyleFinder.first).style;
+    expect(textStyle, same(expectedTextStyle));
+    expect(defaultTextStyle, same(expectedTextStyle));
+  });
 }
 
 Future<void> setWidgetForTooltipMode(