ComboBox with Fluent theme enabled doesn't update its items/dropdown when bound collection is changed

### Description

Binding a collection to ComboBox `ItemsSource` initially correctly sets the dropdown height and adds a scrollbar if needed. However, if bound collection is changed in any way, neither dropdown height nor scrollbar visibility is changed.

This issue is only present when ThemeMode in `App.xaml` is set to any of "Light", "Dark" or "System". Setting ThemeMode to "None" (which effectively disables Fluent theme) reverts to old and correct behavior.

I attached a repro which demonstrates this. Notice that item count and selection continues to function correctly and the only obvious issue is the ComboBox dropdown height not updating whenever the bound collection changes.

### Reproduction Steps

This [repro project](https://github.com/SuperBrain/ComboBoxBugRepro) demonstrates all the details of this issue

### Expected behavior

Any updates to bound collection should immediately update ComboBox dropdown height and scrollbar visibility

### Actual behavior

On collection change, ComboBox dropdown height and scrollbar visibility is not updated

### Regression?

No, this was introduced with first release of Fluent themese support in .NET 9 and is still present with latest release of .NET 10 (currently .NET 10.0.11)

### Known Workarounds

No

### Impact

Severe in all cases, as ComboBox is effectuvely unusable if bound collection has to change after initial binding

### Configuration

- Which version of .NET is the code running on?
.NET 10 v10.0.11 (SDK 10.0.400 14fbf8d527)
- What OS and version, and what distro if applicable?
Windows 11 Pro, 25H2, v10.0.26200.9168, Windows Feature Experience Pack 1000.26100.344.0
- What is the architecture (x64, x86, ARM, ARM64)?
x64
- Do you know whether it is specific to that configuration?
No

### Other information

_No response_

Start with the linked ComboBoxBugRepro project and its App.xaml ThemeMode setting, then reproduce the issue with a ComboBox whose ItemsSource collection changes. Trace the Fluent-themed ComboBox dropdown sizing and scrollbar behavior; done means collection changes update both immediately for Light, Dark, and System themes while preserving the existing None behavior.

Written by the indexing model from the issue text.