This PR introduces DateInputCalendarDelegate, a specialized calendar delegate that extends GregorianCalendarDelegate to support customizable text input behavior in showDatePicker and showDateRangePicker when using DatePickerEntryMode.input.

The default behavior of showDatePicker remains unchanged. This change improves extensibility of the date picker input mode without affecting the calendar UI or existing APIs.

Fixes #83302
Fixes #97134
Fixes #62401
Fixes #181043
Fixes #115151

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• I listed at least one issue that this PR fixes in the description above.
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• 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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