[dkbharti-glitch]

Description

Clarifies how to programmatically clear or reset the selection in DropdownMenu.

Setting initialSelection to null does not clear the text field because it only sets the initial value. This PR documents that providing a TextEditingController and calling controller.clear() alongside updating the state variable to null is required to reset the selection, and adds an interactive code sample demonstrating this pattern.

Fixes flutter/flutter#177599

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• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request adds a new code sample and widget tests demonstrating how to programmatically clear or reset the selection in a DropdownMenu using a TextEditingController. It also updates the documentation for DropdownMenu, its controller property, and its initialSelection property to explain this behavior and reference the new example. A pending changelog entry is also included. There are no review comments, and I have no feedback to provide.