Fix menu width issue for DropdownMenu

[QuncCccccc]

Fixes: #123631.

This PR is to fix the menu width. It should be the same width as the input field.

Borrow the same example that shows in the issue description:)

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].
• All existing and new tests are passing.

[HansMuller]

LGTM

[rydmike]

@QuncCccccc this seems partially a bit unrelated to the issue. It will make the menu unresponsive the general theme VisualDensity changes. Unless the used TextField in the DropdownMenu already is that as well already. Then it makes sense to make sure sizes always line up. Are all the other menus also locked and not changing with VisualDensity? If so (I will check) it makes even more sense. If we can avoid inconsistencies that is always nice. I have heard feedback from Flutter devs that were raging about inconsistencies in VisualDenisity behavior in general in Flutter, but that is another topic.

Nice find with the key, would have taken me a while to figure that out. Thanks for the quick fix.