MultiValueVariable: Fix issue where url update would not take options into account

[mdvictor]

This PR fixes an issue where in a variable with key-value pairs(esc), setting the key in the url param would not update to it's value but instead create a new custom option with the key as a value.

Before:

https://github.com/user-attachments/assets/cf3aca6a-cfc6-4c7f-825d-7f1bf1fc6ff3

After:

https://github.com/user-attachments/assets/575498fe-a385-43c0-a293-bd1d180e5d17

[mdvictor]

I've tested this and indeed it will no longer change that url param from a key to a value, but just keep the value since it exists in the options. It doesn't break anything, but will prioritise the value instead of the label, which I think is okay. So for example if I change the variable value to key C, the url value will update with A, and then refreshing that page will continue to show value A in the url param and key C in the variable value select on the dashboard, which is expected.

[grafanabot]

:rocket: PR was released in v5.10.1 :rocket: