Fix CalendarDatePicker day selection shape and overlay

[TahaTesser]

fixes DatePickerDialog date entry hover background and ink splash have different radius
fixes Ability to customize DatePicker day selection background and overlay shape

Code sample

expand to view the code sample

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Builder(builder: (context) {
            return FilledButton(
              onPressed: () {
                showDatePicker(
                  context: context,
                  initialDate: DateTime.now(),
                  firstDate: DateTime.utc(2010),
                  lastDate: DateTime.utc(2030),
                );
              },
              child: const Text('Show Date picker'),
            );
          }),
        ),
      ),
    );
  }
}

Material DatePicker states specs

Day selection overlay

Before	After

Hover, pressed, highlight preview

Before	After
before_recording.mov	after_recording.mov

Using DatePickerThemeData.dayShape to customize day selection background and overlay shape

Before	After

Example preview

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• 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.
• All existing and new tests are passing.

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[HansMuller]

LGTM with one small request for additional doc and maybe an example.

[HansMuller]

Since this property only appears in the theme (which is OK) you should explain how it interacts with dayBorder and dayForegroundColor

You could also include a simple example here and link the same example from todayBorder and dayForegroundColor.

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Changes reported for pull request #144317 at sha fcf4d843a214e3f085e764d4faa59c7bee0a22b1