symdiff produces unexpected results

Using `symdiff` with all the same types yields expected results
```
symdiff([1u"V", 2u"V", 3u"V"], [3u"V", 4u"V"])
```
yields
```
[1 V, 2 V, 4 V]
```
Changing one of the types breaks it
```
symdiff([1.0u"V", 2u"V", 3u"V"], [3u"V", 4u"V"])
```
yields
```
[1.0 V, 2.0 V, 3.0 V]
```

I'm not sure if the issue is related to julia base or unitful.jl. So I appologize if this is the wrong spot to post this issue. It seems like at minimum this should error out instead of returning incorrect results.

Unitful 1.27
Julia 1.11.7

Reproduce the two examples with Julia 1.11.7 and Unitful 1.27, then trace the `symdiff` implementation and its handling of mixed numeric types. Done means the mixed-type case returns the correct symmetric difference or raises an explicit error, with a regression test covering both examples.

Written by the indexing model from the issue text.